Controlled fluid flow for cleaning an optical element

ABSTRACT

A fluid is directed toward a surface of an optical element based on a first flow pattern, the surface of the optical element including debris and the fluid directed based on the first flow pattern moving at least some of the debris to a first stagnation region at the surface of the optical element; and the fluid is directed toward the optical element based on a second flow pattern, the fluid directed based on the second flow pattern moving at least some of the debris to a second stagnation region on the surface of the optical element, the second stagnation region and the first stagnation region being different locations at the surface of the optical element. Directing the fluid toward the surface of the optical element based on the second flow pattern removes at least some of the debris from the first stagnation region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/937,973, filed Nov. 11, 2015, now allowed, and titled CONTROLLEDFLUID FLOW FOR CLEANING AN OPTICAL ELEMENT, which claims the benefit ofU.S. Provisional Application No. 62/201,955, filed on Aug. 6, 2015, andtitled CONTROLLED FLUID FLOW FOR CLEANING AN OPTICAL ELEMENT. Bothapplications are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The disclosed subject matter relates techniques for controlling a fluidflow to clean an optical element.

BACKGROUND

An optical element includes a surface that interacts with a light beamthrough refraction and/or reflection. The surface can accumulate debrissuch that performance of the optical element is diminished.

SUMMARY

In one general aspect, a fluid is directed toward a surface of anoptical element based on a first flow pattern, the surface of theoptical element including debris and the fluid directed based on thefirst flow pattern moving at least some of the debris to a firststagnation region at the surface of the optical element. The fluid isalso directed toward the optical element based on a second flow pattern,the fluid directed based on the second flow pattern moving at least someof the debris to a second stagnation region on the surface of theoptical element, the second stagnation region and the first stagnationregion being different locations at the surface of the optical element.Directing the fluid toward the surface of the optical element based onthe second flow pattern removes at least some of the debris from thefirst stagnation region.

Implementations can include one or more of the following features. Thefirst and second flow patterns can include information sufficient toindicate a flow of the fluid toward the surface of the optical element,the information including one or more of a flow rate, a spatialdistribution of the fluid relative to the surface of the opticalelement, and a time period during which the fluid is directed toward thesurface of the optical element. The fluid can include a gas. The fluidcan include a free radical. The debris can include a material thatreacts or combines with the material. The free radical can combine orreact with the material to thereby remove the material from the surfacethrough one of combustion of the material, etching of the material, orreaction with the material.

In some implementations, directing the fluid based on the first flowpattern includes directing the fluid toward the surface of the opticalelement at a first flow rate, and directing the fluid based on thesecond flow pattern includes directing the fluid toward the surface ofthe optical element at a second flow rate. The fluid being directedbased on the first flow pattern can include directing the fluid towardthe surface of the optical element in a first direction, and the fluidbeing directed based on the second flow pattern includes directing thefluid toward the surface of the optical element in a second direction.The fluid being directed based on the first flow pattern can includedirecting the fluid toward the surface of the optical element at one ormore of a first flow rate and a first direction, and the fluid beingdirected based on the second flow pattern can include directing thefluid toward the surface of the optical element at one or more of asecond flow rate and a second direction.

Directing the fluid toward the surface based on the first flow patterncan include directing the fluid toward the surface of the opticalelement through a first conduit positioned relative to a first side ofthe optical element at a first flow rate and through a second conduitpositioned relative to a second side of the optical element at a secondflow rate; and directing the fluid toward the surface based on thesecond flow pattern can include directing the fluid toward the surfaceof the optical element through the first conduit at a third flow rateand through the second conduit at a fourth flow rate.

The fluid can be directed through the first conduit and the secondconduit based on the first flow rate during a first time period, and thefluid can be directed through the first conduit and the second conduitbased on the second flow rate during a second time period. The fluid canbe directed through the first conduit and the second conduitconcurrently during at least part of one or more of the first timeperiod and the second time period. The first flow rate and the secondflow rate can be different from each other. The third flow rate and thefourth flow rate can be different from each other, and at least one ofthe third flow rate and the fourth flow rate can be different from thefirst flow rate or the second flow rate. The first flow rate and thesecond flow rate can be the same as each other. The third flow rate andthe fourth flow rate can be the same as each other and different fromthe first flow rate and the second flow rate.

In some implementations, the first flow pattern and the second flowpattern are selected from among a plurality of flow patterns. The fluidcan be directed toward the surface based on the first flow patternduring a first time period, and at least some of the debris accumulatesand remains in the first stagnation region during the first time period.Directing the fluid toward the surface based on the second flow patterncan move at least some of the debris from the first stagnation region tothe second stagnation region.

In some implementations, an initial amount of debris is on the surfaceof the optical element prior to directing the fluid toward the surfacebased on the first flow pattern, and an amount of debris on the surfaceafter the fluid is directed toward the surface of the optical elementbased on the first flow pattern is less than the initial amount ofdebris.

The debris can include a metal. The debris can include tin.

The first stagnation region and the second stagnation region can benon-overlapping regions at the surface of the element. The firststagnation region and the second stagnation region can be partiallyoverlapping regions at the surface of the element.

A second fluid can be directed through the optical element.

In another general aspect, a system includes a conduit including asidewall, the sidewall including at least one opening that passes froman interior of the conduit to an exterior of the conduit, the conduitconfigured to transport free radicals in the interior of the conduit andto pass the free radicals through the at least one opening that passesthrough the sidewall; and a control system including an electronicstorage coupled to one or more electronic processors, the electronicstorage including instructions that, when executed, cause the one ormore electronic processors to: select a flow pattern from a plurality offlow patterns stored on the electronic storage, each of the flowpatterns including information sufficient to describe a flow of the freeradicals through the at least one opening, the information sufficient todescribe the flow of the free radicals through the at least one openingincluding at least a flow rate of the free radicals over a time period,and apply the selected flow pattern to a flow controller of the systemto cause the free radicals to pass through the at least one opening at aflow rate for the time period indicated by the selected flow pattern.

Implementations can include one or more of the following features. Theflow patterns can also include gas flow patterns, the gas flow patternsincluding information sufficient to describe a flow of gas, the gasflowing in the system separately from the conduit, and the informationsufficient to describe the flow of the gas including a flow rate of thegas over a gas time period, and the instructions can also includeinstructions that, when executed, cause the one or more processors toapply the selected gas flow pattern to the flow controller of the systemto cause the gas to flow in the system at a gas flow rate for a gas timeperiod indicated by the selected gas flow pattern.

The conduit can be configured for placement relative to an opticalelement in a vacuum chamber of an extreme ultraviolet (EUV) lightsource. The optical element can include a collector mirror, thecollector mirror having an aperture, the selected flow pattern includesa selected gas flow pattern, and applying the selected gas flow patternto the flow controller of the system can cause the gas to flow throughthe aperture of the collector mirror at the gas flow rate and during thegas time period indicated by the selected gas flow pattern. In someimplementations, the conduit includes at least two curved portions, thecurved portions having a curvature that follows a perimeter of thecollector mirror, the selected flow pattern describes the flow of freeradicals through each of the at least two curved portions, applying theselected gas flow pattern causes the gas to flow through the aperture ofthe mirror, and around a perimeter of the mirror, and applying theselected flow pattern causes the free radicals to flow from at least oneof the at least two curved portions.

In another general aspect, a first flow pattern is accessed, the firstflow pattern including information sufficient to information sufficientto describe a flow of free radicals and a gas relative to an opticalelement, free radicals are directed toward the optical element based onthe first flow pattern, the free radicals being directed through aplurality of paths and at different flow rates in at least two of thepaths; and the gas is directed toward the optical element based on thefirst flow pattern, the gas being directed along a path that isdifferent from any of the paths along which the free radicals aredirected.

Implementations of any of the techniques described above may include atransport system for a laser produced plasma EUV light source, an EUVlight source, a system for retrofitting an EUV light source, a method, aprocess, a device, executable instructions stored on a computer readablemedium, or an apparatus. The details of one or more implementations areset forth in the accompanying drawings and the description below. Otherfeatures will be apparent from the description and drawings, and fromthe claims.

DRAWING DESCRIPTION

FIGS. 1A and 1B are side views of an example of an optical element attwo different times.

FIGS. 1C and 1D are top views of the optical element of FIGS. 1A and 1B,respectively.

FIGS. 1E and 1F are perspective views of the optical element of FIGS. 1Aand 1B, respectively.

FIG. 2 is a block diagram of an example of a system that includes anoptical system, which uses an optical element such as the opticalelement of FIGS. 1A-1F.

FIG. 3 is a flow chart of an example of a process for cleaning anoptical element.

FIG. 4A is a top view of an example of an optical element and conduits.

FIG. 4B is a partial perspective view of one of the conduits of FIG. 4A.

FIG. 4C is a side cross-sectional view of the optical element andconduits of FIG. 4A.

FIGS. 5A and 5B are perspective views of the optical element andconduits of FIG. 4A during a cleaning cycle.

FIG. 6A is a block diagram of an example of a laser produced plasmaextreme ultraviolet (EUV) light source.

FIG. 6B is a block diagram of an example of a drive laser system thatcan be used in the light source of FIG. 6A.

FIG. 7A-7C show a side view of an example of a free radical transportsystem at three different times.

FIG. 8A is a side view of a block diagram of an example of a freeradical transport system.

FIG. 8B is a view of the system of FIG. 8A taken along line 8B-8B.

FIG. 8C is a view of the system of FIG. 8A taken along line 8C-8C.

FIG. 9 is a flow chart of an example process for cleaning an element.

FIGS. 10-12 and 13A are perspective views of examples of conduits fortransporting free radicals.

FIG. 13B is a side view of an example of free radical transport system.

FIG. 14A is a side view of an example of free radical transport system.

FIG. 14B is a view of the system of FIG. 14A taken along line 14B-14B.

FIG. 15A is a view of an example of a free radical transport system.

FIG. 15B is a perspective view of a conduit that can be used in thesystem of FIG. 15A.

FIG. 16 is a view of an example of a free radical transport system.

FIG. 17 is a block diagram of an example of a EUV light source.

FIG. 18A is a front view of an example of a fluid transport system.

FIG. 18B is a side view of the system of FIG. 18A.

FIG. 19 is a flow chart of an example of a process for cleaning anoptical element.

DETAILED DESCRIPTION

Techniques for controlling a flow of fluid to clean an optical elementthat is a part of and operates within an optical system are disclosed.The techniques allow the optical element to be cleaned without removingthe element from its operating environment within the optical system.

In some implementations, the optical element can be cleaned while theoptical system is operating. In this way, the techniques discussedherein can lead to improved performance and efficiency of the opticalsystem.

Referring to FIGS. 1A-1F, an optical element 120 is shown. FIGS. 1A and1B show the optical element 120 from the side at two different times, t1and t2, respectively. FIGS. 1C and 1D are top views of the opticalelement 120 at the times t1 and t2, respectively. FIGS. 1E and 1F show aperspective view of the optical element at the times t1 and t2,respectively.

The optical element 120 can be any type of element or apparatus that iscapable of interacting with an optical (or light) beam of the opticalsystem. For example, the optical element 120 can be a mirror, lens, aprism, or a combination of such components. The optical element 120includes a surface 122 that interacts with the optical beam throughreflection and/or refraction.

The surface 122 can accumulate debris 124, which can hinder theperformance of the optical element 120. The debris 124 can be, forexample, dust and/or metallic particles. The optical element 120 can bepart of an optical system such as an extreme ultraviolet (EUV) lightsource that produces EUV light by converting a metal target to plasma.In addition to producing EUV light, the plasma production process canalso produce debris in the form of particles, vapor residue, or piecesof matter (such as metal) that is in the target. This debris canaccumulate on the surfaces of objects in the path of the plasma. Whenthe element 120 is used in an EUV light source, the debris 124 caninclude these plasma-related products.

During a cleaning cycle, a fluid 105 is directed toward the surface 122to clean the optical element 120 by removing the debris 124 from thesurface 122. For example, the fluid 105, or a substance carried by thefluid 120, can react or combine with the debris 124 to remove the debris124 from the surface 122. The fluid 105 also can remove the debris fromthe surface 122 by physical force.

The fluid 105 flows or moves relative to the surface 122. Thus, inaddition to removing debris from the surface 122, if all of the debris124 is not removed, the fluid 105 can transport the remaining debris 124to another portion of the surface 122. The transported debris can becometrapped or accumulate in certain locations on the surface 122, and theselocations are referred to as “stagnation regions.” The location of thestagnation region or regions depends on the characteristics of the flowof the fluid 105 (for example, the flow rate and/or direction) andcharacteristics of the surface 122 (for example, the shape of thesurface 122 and the material of the surface 122). The characteristics ofthe flow of the fluid 105 are described by a flow pattern, which caninclude information such as, for example, flow rate, flow direction,and/or a time for which the fluid 105 is directed at the surface 122 ata particular flow rate and in a particular direction.

Because the location of the stagnation region or regions depends on thecharacteristics of the flow of the fluid 105, changing the flow patternof the fluid 105 during the cleaning cycle can result in the stagnationregion or regions moving to different locations on the surface 122. Bymoving the stagnation region, debris trapped in a stagnation regionformed by one flow pattern is freed and can be removed by the fluid 105that flows based on another flow pattern. In this way, cleaning thesurface 122 becomes more effective and faster.

In the example shown in FIGS. 1A, 1C, and 1E, the fluid 105 is directedtoward the surface 122 based on a first flow pattern. In other words,the fluid 105 is directed toward the surface 122 at a flow rate and/ordirection relative to the surface 122 as dictated or indicated byinformation that defines the first flow pattern. The interaction betweenthe fluid 105 and the debris 124 can remove some of the debris 124 fromthe surface 122, and the fluid 105 also transports some of the debris124 to other parts of the surface 122. Of the debris 124 that is notremoved from the surface 122, a relatively large amount of thetransported debris accumulates at first stagnation regions 125.

Referring to FIGS. 1B, 1D, and 1F, at the time t2, the fluid 105 isdirected toward the surface 122 based on a second flow pattern, which isa different flow pattern than the first flow pattern. In the example ofFIGS. 1B and 1D, the second flow pattern directs the fluid 105 towardthe surface 122 in a different direction as compared to the first flowpattern. Directing the fluid 105 toward the surface 122 based on thesecond flow pattern removes some of the debris 124 from the surface 122.Further, because the fluid 105 flows relative to the surface 122 in adifferent direction than it does at the time t1, the fluid 105 interactswith accumulated debris in the first stagnation region 125. Through theinteraction, the fluid 105 removes debris from the first stagnationregion 125.

Additionally, the fluid 105 can transport some of the debris on thesurface 122 (including debris in the first stagnation region 125) toanother location on the surface 122. Some the transported debris canaccumulate in a second stagnation region 126. The second stagnationregion 126 is in a different location on the surface 122 than the firststagnation region 125 in that at least some of the second stagnationregion 126 includes a part of the surface 122 that is not included inthe first stagnation region 125. Thus, the first and second stagnationregions 125, 126 can be regions that partially spatially overlap or canbe regions that do not share any common spatial points on the surface122.

Referring to FIG. 2, a block diagram of an example of a system 200 thatincludes an optical system 203, which uses the optical element 120. Thesystem 200 includes a cleaning system 201 that directs the fluid 105toward the optical element 120 to clean the surface 122 of the opticalelement 120. The optical element 120, which can be any component capableof interacting with optical energy or an optical (light) beam, is partof an optical system 203. The optical system 203 can be any type ofsystem that uses the optical element 120 to manipulate, produce, and/orinteract with optical energy. For example, the optical system 203 can bea laser, an optical amplifier, or an imaging system. The optical system203 can be a light source used to produce extreme ultraviolet (EUV)light. An example of an implementation in which the optical system 203is an EUV light source are discussed with respect to FIG. 17.

The cleaning system 201 includes a control system 255 that controls afluid transport system 202. The fluid transport system 202 includes afluid supply 210, which provides the fluid 105. In some implementations,the fluid transport system 202 also includes a conduit 250 that iscoupled to the fluid supply 210 and receives the fluid 105 from thefluid supply 210. The conduit 250 can transport some or all of the fluid105 to the optical system 203.

The fluid 105 can include free radicals, and, in these implementations,the fluid supply 210 includes a source of free radicals. Inimplementations in which the fluid 205 includes free radicals, the fluidsupply 210 can be a microwave plasma generator. To produce free radicalswith such a source, a gas from which the free radicals are formed isprovided to the fluid supply 210. For example, to produce hydrogenradicals, hydrogen gas (H₂) is provided. An additional gas can be addedto the gas prior to providing the gas to the fluid supply 210. Forexample, a mixture of argon and oxygen (Ar/O₂) can be added to thehydrogen gas.

Alternatively or additionally, the fluid supply 210 also can include asource of gas that does not include free radicals. For example, thefluid supply 210 can provide diatomic molecular hydrogen (H₂) gas,helium gas (He), and/or argon gas (Ar). The fluid supply 210 can includemore than one source of gas. For example, the fluid supply 210 caninclude two separate sources of gas.

The cleaning system 201 also includes the control system 255. Thecontrol system 255 controls the flow of the fluid 105. For example, thecontrol system 255 can apply a particular flow pattern or a sequence offlow patterns to indicate how the fluid 105 is delivered to the opticalsystem 203.

The example control system 255 shown in FIG. 2 includes a conduitcontroller 240, a flow controller 241, an electronic processor 242, anelectronic storage 243, and an input/output (I/O) interface 244. Inimplementations that include the conduit 250, the conduit controller 240controls the position of the conduit 250 relative to the optical element120. The conduit controller 240 can change or set the distance betweenthe conduit 250 and the optical element 120 and/or can rotate theconduit 250 relative to the optical element 120. The conduit controller240 can be used to position the conduit 250 relative to the opticalelement 120 when the conduit 250 is initially installed in the system200, after the conduit 250 has been installed in the system 200, and/orwhile the system 200 is in use.

The conduit controller 240 can include components that areelectronically controlled. For example, the conduit 250 can be coupledto a stepper motor or other device that moves the conduit 250 whenactivated by a user or an automated electronic process. The conduitcontroller 240 can include components that are configured for manualoperation, for example, with a lever, wheel, or other mechanical devicethat is accessible from outside of the system 200 and allows an operatorof the system 200 to move the conduit 250.

In some implementations, the control system 255 does not include theconduit controller 240. For example, in some implementations thatinclude the conduit 250, the conduit 250 is permanently positionedrelative to the optical element 120 and is not moved after installationor during use. In these implementations, the control system 255 can lackthe conduit controller 240 or the conduit controller 240 can bedisabled.

The flow controller 241 is used to control the flow of the fluid 105.The flow controller 241 can include a mass flow controller that measuresand/or controls the flow of the fluid 105 and a pressure controller thatcontrols the relative pressure between the fluid supply 210 and theoptical system 203.

The electronic processor 242 is one or more processors suitable for theexecution of a computer program such as a general or special purposemicroprocessor, and any one or more processors of any kind of digitalcomputer. Generally, a processor receives instructions and data from aread-only memory or a random access memory or both. The electronicprocessor 242 can be any type of electronic processor.

The electronic storage 243 can be volatile memory, such as RAM, ornon-volatile memory. In some implementations, the electronic storage 243can include both non-volatile and volatile portions or components. Theelectronic storage 243 stores instructions, perhaps as a computerprogram, that, when executed, cause the processor 242 to communicatewith other components in the control system 255.

The electronic storage 243 also includes a flow pattern store 246. Theinformation sufficient to define a plurality of flow patterns is storedin the flow pattern store 246. The information in each of the flowpatterns in the flow pattern store 246, when applied to the fluidtransport system 202 by the control system 255, causes the fluid 205 tobe delivered to the optical element 120 in a manner that is specified bythe flow pattern. The control system 255 can receive an input from ahuman operator or an automated electronic process at the I/O interface244 requesting that a particular flow pattern, or sequence of flowpatterns that form a “recipe,” be applied to the fluid transport system202. The control system 255 can implement the various flow patterns by,for example, communicating a command to the flow controller 241 to causethe mass flow controller to measure or determine the current flow rateof the fluid 205 and adjust the flow rate to match the flow ratespecified by the flow pattern.

The flow patterns stored in the flow pattern store 246 can be stored inassociation with one or more specific configurations of the opticalsystem 203 and/or with one or more optical systems. In this way, anoperator of the system 200 (or an automated process) can select a flowpattern or a series of flow patterns that are optimized for a particularoptical system and/or configuration.

The I/O interface 244 is any kind of electronic interface that allowsthe control system 255 and/or its components to receive and/or providedata and signals to an operator and/or an automated process running onanother electronic device. For example, the I/O interface 244 caninclude one or more of a touch screen or a communications interface.

Referring to FIG. 3, a flow chart of an example process 300 is shown.The process 300 is discussed with respect to the system 200 of FIG. 2.However, the process 300 can be performed by other systems. The process300 can be implemented as instructions stored on the electronic storage243 and performed by the electronic processor 242 and the othercomponents of the control system 255. In some implementations, theprocess 300 can be stored on a separate tangible computer-readablemedium and used to retrofit or upgrade an existing system.

The fluid 105 is directed toward the optical element 120 based on afirst flow pattern (310). The optical element 120 includes the surface122, and the fluid 105 can be directed toward the surface 122 of theoptical element 120. The first flow pattern is information that defineshow the fluid 105 interacts with the optical element 120. For example,the first flow pattern can indicate a flow rate, a spatial distributionof the fluid 105 relative to the optical element 120, and/or a directionof flow of the fluid 105 relative to the optical element 120.

The fluid 105 can include free radicals. A free radical is an atom,molecule, or ion that has an unpaired valence electron or an openelectron shell, and, therefore, may be seen as having a danglingcovalent bond. The dangling bonds can make free radicals highlychemically reactive, that is, a free radical can react readily withother substances. Because of their reactive nature, free radicals can beused to remove a substance (such as the debris 124) from an object (suchas the surface 122 of the optical element 120). The free radicals canremove the debris by, for example, etching, reacting with and/orcombusting the debris.

The fluid 105 can be a gas that does not include free radicals. In theseimplementations, the fluid supply 210 can provide, for example, diatomicmolecular hydrogen (H₂) gas, helium gas (He), and/or argon gas (Ar). Thefluid 105 also can be a mixture of free radicals and a gas. In someimplementations, the fluid 105 includes both free radicals and gas, andthe free radicals and gas are directed toward the optical element 120through different paths or mechanisms. The neutral gas can be a carriergas for transporting the radicals to destinations.

The fluid 105 is directed toward the optical element 120 based on asecond flow pattern (320). The fluid 105 can be directed toward thesurface 122 of the optical element 120. The second flow pattern isdifferent from the first flow pattern. For example, the fluid 105 canhave a different flow rate, direction, spatial pattern, and/or durationwhen directed toward the optical element 120 based on the second flowpattern as compared to the first flow pattern. The fluid 105 flowsrelative to the surface 122 and removes debris 124 from the surface 122.The fluid 105 also can transport some of the debris 124 relative to thesurface 122, and a portion of the transported debris can accumulate inthe second stagnation region 126.

The fluid 105 is directed toward the optical element 120 in any manner.For example, the fluid 105 can be generated inside of the optical system203 and provided to the optical element 120 without a structure totransport the fluid 105. In some implementations, all or some of thefluid 105 can be delivered to the optical element 120 with the conduit250.

Referring also to FIGS. 4A-4C, an optical element 420 and a two-piececonduit 450 are shown. The conduit 450 includes a first conduit 450 aand a second conduit 450 b. The conduits 450 a, 450 b are positioned ata perimeter 427 of the optical element 420. In the discussion below, theoptical element 420 is in the optical system 203, and the fluid 105 isdelivered to the optical element 420 through the conduit 450.

FIG. 4A is a top view of the optical element 420 and the conduits 450 a,450 b. FIG. 4B is a partial perspective view of the conduit 450 a andapertures 454 through which the fluid 105 exits the conduit 450 a. Theconduit 450 b also includes a plurality of apertures through which thefluid 105 flows. FIG. 4C is a side cross-sectional view of the opticalelement 420 and the conduits 450 a, 450 b. The optical element 420includes a surface 422, which interacts with light through reflectionand/or reflection. The surface 422 of the optical element 420 is curvedin a concave shape in the x-z plane (FIG. 4C). In this example, thefluid supply 210 (FIG. 2) includes two separate fluid supplies 210 a and210 b, one connected to each of the conduits 450 a and 450 b.

FIGS. 5A and 5B are perspective views of the optical element 420 andpartial perspective views of the conduits 450 a, 450 b while the fluid105 flows toward the optical element 420 based on the flow pattern A andthe second flow pattern B, respectively.

FIGS. 5A and 5B illustrate the fluid 105 flowing during a cleaning cyclegoverned by the process 300 to clean the optical element 420. In thecleaning cycle illustrated in FIGS. 5A and 5B, the fluid 105 is directedtoward the optical element 420 based on two different flow patterns, aflow pattern A (represented by solid arrows) that is applied during atime period A, and a flow pattern B (represented by dashed arrows) thatis applied during a time period B. In this example, the flow patterns Aand B define a recipe for cleaning the optical element 420 in a singlecleaning cycle. The cleaning cycle occurs while the optical element 420is in the optical system 203. Although in this example the cleaningcycle includes a plurality of flow patterns, in some implementations,the cleaning cycle includes a single flow pattern.

Referring to FIG. 5A, the fluid 105 is directed toward the opticalelement 420 based on the flow pattern A. Information that describes theflow pattern A is stored on the flow pattern store 246 of the cleaningsystem 201, and the information is retrieved by the control system 255and applied to the fluid transport system 202. In this example, theinformation that describes the flow pattern A causes the fluid 105 to bedirected through the conduit 450 a and the conduit 450 b for time periodthat is also indicated by the information that describes the flowpattern A.

The information that describes the flow pattern A can be, for example,instructions stored on the electronic storage 243 that, when executed bythe electronic processor 242, cause the fluid supply 210 a to deliverthe fluid 105 into the conduit 450 a at a first flow rate and cause thefluid supply 210 b to deliver the fluid 105 into the conduit 450 b at asecond flow rate. The first and second flow rates are individuallycontrollable, and can be the same flow rate or different flow rates. Theinformation that defines the flow pattern A can include specific flowrates at which the fluid 105 is to flow in the conduits 450 a and 450 b,or the flow pattern A can specify a ratio between the flow rates thatthe flow 105 is to flow in the conduit 450 a relative to the flow ratein the conduit 450 b.

Although in this example the information that describes the flow patternA results in the fluid 105 flowing from both the conduit 450 a and theconduit 450 b, in other examples the fluid 105 can flow from one of theconduits 450 a, 450 b.

The information that describes the flow pattern A also can define thedirection in which the fluid 105 flows relative to the surface 422. Forexample, the information that describes the flow pattern A can includeinstructions that cause the conduit controller 240 to move the conduit450 a and/or the conduit 450 b relative to the surface 422 to direct thefluid 105 in a particular direction. The instructions can cause theconduit 450 a and/or the conduit 450 b to move by translating theconduit 450 a and/or 450 b relative to the surface 122 in the x-y plane,moving the conduit 450 a and/or 450 b closer to or farther from thesurface 122 in the x-z or y-z planes, and/or by rotating the conduit 450a and/or 450 b and the opening 454 relative to the surface 122.

The information that describes the first flow pattern also can determinehow long the fluid 105 is directed toward the optical element 420 basedon the flow pattern A. In the example of FIG. 5A, the fluid 105 isdirected toward the optical element 420 for the time period A, which canalso be specified by the information that defines the flow pattern A.The time period A can be, for example, minutes or hours.

When directed toward the surface 122 based on the first flow pattern,the fluid 105 moves relative to the surface 422, removing debris 424from the surface 422 and also transporting some of the debris 424 toother portions of the surface 422. The combination of the flow of thefluid 105 and the shape of the surface 422 causes a large portion of thedebris that is not removed from the surface 422 to become trapped in astagnation region that is at a location 425 on the surface 422.

Referring to FIG. 5B, the fluid 105 is directed toward the opticalelement 420 based on the flow pattern B. Directing the fluid 105 basedon the flow pattern B causes the stagnation region to move, allowing thefluid 105 to remove the debris that became trapped at the location 425.

Like the information that defines the flow pattern A, the informationthat defines the flow pattern B can include, for example, flow rates forthe fluid 105, a direction of flow for the fluid 105 that exits theconduit 450 a and/or 450 b, and/or a duration of time (time period B)during which the fluid 105 flows toward the surface 422 based on theflow pattern B. Some or all of these parameters of flow pattern B can bedifferent from the parameters of flow pattern A. For example, flowpattern B can be defined by information that specifies that the flowrates in conduit 450 a and 450 b are different, and flow pattern A canbe defined by information that specifies that the flow rates in 450 aand 450 b are the same.

Thus, the information that describes the flow pattern B causes the fluid105 to flow toward the surface 422 in a different manner as compared towhen the fluid is directed toward the surface 422 based on the flowpattern A. By changing the characteristics of the flow of the fluid 105,debris trapped in the stagnation region located on the surface 422 atlocation 425 during the time period A (associated with the flow patternA) can be removed from the surface 422 of the optical element 420. Thedebris is removed from the surface 422 while the optical element 420 isin the optical system 203. The fluid 105 can be directed toward theoptical element 422 while the optical system 203 is in use (for example,while the surface 422 interacts with light). Thus, the procedure 300allows the optical element 420 to be cleaned with minimal to no downtimeof the system 200.

The fluid 105 that flows based on the flow pattern B also can create astagnation region in a different location of the surface than thelocation 425.

As discussed above, the optical system 203 can be an EUV light source,and the optical element 120 or 420 can be an optical element in the EUVlight source. The fluid 105 can include free radicals that react withthe debris on the surface of the optical element to remove the debrisfrom the surface. Additionally, the fluid 105 can include gases that donot include free radicals. Further, other gases and/or fluids can bedirected toward the optical element to enhance the removal of debrisfrom the optical element. The fluid 105 and/or other fluids can bedirected toward the optical element through a conduit or through anopening or passageway formed in physical element or between a pluralityphysical elements.

Various examples of conduits, systems, and configurations in which acleaning fluid can flow based on a flow patterns are discussed below. Anexample of an EUV light source 600 is discussed with respect to FIGS. 6Aand 6B. FIGS. 7A-7C, 8A-8C, 9, 11B, 12A and 12B, and 13 relate toexamples of free radical transport systems that can be used to deliverfree radicals to an element in the EUV light source 600 or in other EUVlight sources. FIGS. 10-12 and 11A show examples of conduits thattransport free radicals (and these conduits can be used as the conduit250 in the system 200). FIG. 17 shows an example of another EUV lightsource, and FIGS. 18A and 18B show an example of fluid transport system.

Before discussing the flow patterns in more detail, an EUV light sourcediscussed.

Referring to FIG. 6A, an LPP EUV light source 600 is shown. The LPP EUVlight source 600 includes a free radical transport system 700. Thetransport system 600 is shown as being part of the source 600. However,the transport system 700 can be removed from and reinserted into thesource 600. The EUV light source 600 is discussed before discussing thefree radical transport system 700. The free radical transport system 700is discussed in greater detail beginning with FIG. 7A.

The LPP EUV light source 600 is formed by irradiating a target mixture614 at a target location 605 with an amplified light beam 610 thattravels along a beam path toward the target mixture 614. The targetlocation 605, which is also referred to as the irradiation site, iswithin an interior 607 of a vacuum chamber 630. When the amplified lightbeam 610 strikes the target mixture 614, a target material within thetarget mixture 614 is converted into a plasma state that has an elementwith an emission line in the EUV range. The created plasma has certaincharacteristics that depend on the composition of the target materialwithin the target mixture 614. These characteristics can include thewavelength of the EUV light produced by the plasma and the type andamount of debris released from the plasma.

The light source 600 also includes a target material delivery system 625that delivers, controls, and directs the target mixture 614 in the formof liquid droplets, a liquid stream, solid particles or clusters, solidparticles contained within liquid droplets or solid particles containedwithin a liquid stream. The target mixture 614 includes the targetmaterial such as, for example, water, tin, lithium, xenon, or anymaterial that, when converted to a plasma state, has an emission line inthe EUV range. For example, the element tin can be used as pure tin(Sn); as a tin compound, for example, SnBr4, SnBr2, SnH4; as a tinalloy, for example, tin-gallium alloys, tin-indium alloys,tin-indium-gallium alloys, or any combination of these alloys. Thetarget mixture 614 can also include impurities such as non-targetparticles. Thus, in the situation in which there are no impurities, thetarget mixture 614 is made up of only the target material. The targetmixture 614 is delivered by the target material delivery system 625 intothe interior 607 of the chamber 630 and to the target location 605.

The light source 600 includes a drive laser system 615 that produces theamplified light beam 610 due to a population inversion within the gainmedium or mediums of the laser system 615. The light source 600 includesa beam delivery system between the laser system 615 and the targetlocation 605, the beam delivery system including a beam transport system620 and a focus assembly 622. The beam transport system 620 receives theamplified light beam 610 from the laser system 615, and steers andmodifies the amplified light beam 610 as needed and outputs theamplified light beam 610 to the focus assembly 622. The focus assembly622 receives the amplified light beam 610 and focuses the beam 610 tothe target location 605.

In some implementations, the laser system 615 can include one or moreoptical amplifiers, lasers, and/or lamps for providing one or more mainpulses and, in some cases, one or more pre-pulses. Each opticalamplifier includes a gain medium capable of optically amplifying thedesired wavelength at a high gain, an excitation source, and internaloptics. The optical amplifier may or may not have laser mirrors or otherfeedback devices that form a laser cavity. Thus, the laser system 615produces an amplified light beam 610 due to the population inversion inthe gain media of the laser amplifiers even if there is no laser cavity.Moreover, the laser system 615 can produce an amplified light beam 610that is a coherent laser beam if there is a laser cavity to provideenough feedback to the laser system 615. The term “amplified light beam”encompasses one or more of: light from the laser system 615 that ismerely amplified but not necessarily a coherent laser oscillation andlight from the laser system 615 that is amplified and is also a coherentlaser oscillation.

The optical amplifiers in the laser system 615 can include as a gainmedium a filling gas that includes CO2 and can amplify light at awavelength of between about 9100 and about 11000 nm, and in particular,at about 10600 nm, at a gain greater than or equal to 600. Suitableamplifiers and lasers for use in the laser system 615 can include apulsed laser device, for example, a pulsed, gas-discharge CO2 laserdevice producing radiation at about 9300 nm or about 10600 nm, forexample, with DC or RF excitation, operating at relatively high power,for example, 10 kW or higher and high pulse repetition rate, forexample, 40 kHz or more. The optical amplifiers in the laser system 615can also include a cooling system such as water that can be used whenoperating the laser system 615 at higher powers.

FIG. 6B shows a block diagram of an example drive laser system 680. Thedrive laser system 680 can be used as the drive laser system 615 in thesource 600. The drive laser system 680 includes three power amplifiers681, 682, and 683. Any or all of the power amplifiers 681, 682, and 683can include internal optical elements (not shown).

Light 684 exits from the power amplifier 681 through an output window685 and is reflected off a curved mirror 686. After reflection, thelight 684 passes through a spatial filter 687, is reflected off of acurved mirror 688, and enters the power amplifier 682 through an inputwindow 689. The light 684 is amplified in the power amplifier 682 andredirected out of the power amplifier 682 through an output window 690as light 691. The light 691 is directed toward the amplifier 683 with afold mirror 692 and enters the amplifier 683 through an input window693. The amplifier 683 amplifies the light 691 and directs the light 691out of the amplifier 683 through an output window 694 as an output beam695. A fold mirror 696 directs the output beam 695 upward (out of thepage) and toward the beam transport system 620 (FIG. 6A).

Referring again to FIG. 6B, the spatial filter 687 defines an aperture697, which can be, for example, a circle having a diameter between about2.2 mm and 3 mm. The curved mirrors 686 and 688 can be, for example,off-axis parabola mirrors with focal lengths of about 1.7 m and 2.3 m,respectively. The spatial filter 687 can be positioned such that theaperture 697 coincides with a focal point of the drive laser system 680.

Referring again to FIG. 6A, the light source 600 includes a collectormirror 635 having an aperture 640 to allow the amplified light beam 610to pass through and reach the target location 605. The collector mirror635 can be, for example, an ellipsoidal mirror that has a primary focusat the target location 605 and a secondary focus at an intermediatelocation 645 (also called an intermediate focus) where the EUV light canbe output from the light source 600 and can be input to, for example, anintegrated circuit lithography tool (not shown). The light source 600can also include an open-ended, hollow conical shroud 650 (for example,a gas cone) that tapers toward the target location 605 from thecollector mirror 635 to reduce the amount of plasma-generated debristhat enters the focus assembly 622 and/or the beam transport system 620while allowing the amplified light beam 610 to reach the target location605. For this purpose, a gas flow can be provided in the shroud that isdirected toward the target location 605.

The light source 600 can also include a master controller 655 that isconnected to a droplet position detection feedback system 656, a lasercontrol system 657, and a beam control system 658. The light source 600can include one or more target or droplet imagers 660 that provide anoutput indicative of the position of a droplet, for example, relative tothe target location 605 and provide this output to the droplet positiondetection feedback system 656, which can, for example, compute a dropletposition and trajectory from which a droplet position error can becomputed either on a droplet by droplet basis or on average. The dropletposition detection feedback system 656 thus provides the dropletposition error as an input to the master controller 655. The mastercontroller 655 can therefore provide a laser position, direction, andtiming correction signal, for example, to the laser control system 657that can be used, for example, to control the laser timing circuitand/or to the beam control system 658 to control an amplified light beamposition and shaping of the beam transport system 620 to change thelocation and/or focal power of the beam focal spot within the chamber630.

The target material delivery system 625 includes a target materialdelivery control system 626 that is operable, in response to a signalfrom the master controller 655, for example, to modify the release pointof the droplets as released by a target material supply apparatus 627 tocorrect for errors in the droplets arriving at the desired targetlocation 605.

Additionally, the light source 600 can include light source detectors665 and 670 that measures one or more EUV light parameters, includingbut not limited to, pulse energy, energy distribution as a function ofwavelength, energy within a particular band of wavelengths, energyoutside of a particular band of wavelengths, and angular distribution ofEUV intensity and/or average power. The light source detector 665generates a feedback signal for use by the master controller 655. Thefeedback signal can be, for example, indicative of the errors inparameters such as the timing and focus of the laser pulses to properlyintercept the droplets in the right place and time for effective andefficient EUV light production.

The light source 600 can also include a guide laser 675 that can be usedto align various sections of the light source 600 or to assist insteering the amplified light beam 610 to the target location 605. Inconnection with the guide laser 675, the light source 600 includes ametrology system 624 that is placed within the focus assembly 622 tosample a portion of light from the guide laser 675 and the amplifiedlight beam 610. In other implementations, the metrology system 624 isplaced within the beam transport system 620. The metrology system 624can include an optical element that samples or re-directs a subset ofthe light, such optical element being made out of any material that canwithstand the powers of the guide laser beam and the amplified lightbeam 610. A beam analysis system is formed from the metrology system 624and the master controller 655 since the master controller 655 analyzesthe sampled light from the guide laser 675 and uses this information toadjust components within the focus assembly 622 through the beam controlsystem 658.

Thus, in summary, the light source 600 produces an amplified light beam610 that is directed along the beam path to irradiate the target mixture614 at the target location 605 to convert the target material within themixture 614 into plasma that emits light in the EUV range. The amplifiedlight beam 610 operates at a particular wavelength (that is alsoreferred to as a drive laser wavelength) that is determined based on thedesign and properties of the laser system 615. Additionally, theamplified light beam 610 can be a laser beam when the target materialprovides enough feedback back into the laser system 615 to producecoherent laser light or if the drive laser system 615 includes suitableoptical feedback to form a laser cavity.

FIGS. 7A-7C show block diagrams of an example of a free radicaltransport system 700 at three different times, time1 (t1), time2 (t2),and time3 (t3), respectively. Time t1 is the earliest time, with time t2occurring after time t1, and time t3 occurring after time t2. The freeradical transport system 700 delivers free radicals 705 to an element720 that is inside of a vessel 730. The free radical transport system700 is also shown in FIG. 6A. Additionally, the free radical transportsystem 700 can be used in the system 200 (FIG. 2) as the fluid transportsystem 202.

The free radical transport system 700 includes a conduit 750 thatconnects to a source 710 of free radicals 705. The free radicals 705flow into an opening 752 defined by the conduit 750 and travel in theconduit 750 along a direction 707. The conduit 750 also defines anotheropening 754 that passes through a sidewall 756 of the conduit 750 toprovide a passage between an interior 758 of the conduit 750 and theinterior 732 of the vessel 730. The vessel 730 can be a vacuum chamber,such as the vacuum chamber 630 discussed above. The element 720 can beany element that is exposed to debris that is generated in the interior732 of the vessel 730. The element 720 can be an optical element that isin the path of plasma that is generated in the interior 732. Forexample, the element 720 can be a collector mirror, such as thecollector mirror 635 of FIG. 6A.

Referring to FIG. 7A, the free radicals 705 travel from a source 710through the conduit 750 in the direction 707 and exit through theopening 754 into the interior 732. The opening 754 is positioned so thatthe radicals 705 flow to the element 720 after exiting through theopening 754. For example, the opening 754 can be positioned to face asurface 722 of the element 720. The opening 754 is positioned a distance726 from the surface 722. The distance 726 can be, for example, 15 to 30cm. In the example shown, the conduit 750 includes the one additionalopening 754, however, in other implementations, such as those shown inFIGS. 8A-8C and 10-16, a plurality of openings are formed in theconduit.

The element 720 is located in an interior 732 of the vessel 730. Theconduit 750 passes through a sealed opening or port 734 in a wall 736 ofthe vessel 730. Thus, the conduit 750 transports the free radicals 705from the external source 710 to the element 720. The free radicals 705exit the conduit 750 through the opening 754 and pass into the interior732.

Referring to FIG. 7B, the radicals 705 reach debris 724 that is on thesurface 722. Continuing the example of the element 720 being in the pathof plasma that is generated in the interior 732, the debris 724 can becontamination that originates from vapors, ions, particles, and/orclusters formed from a target mixture used to generate the plasma. Thetarget mixture can be any material that emits EUV light when convertedto plasma. Thus, the debris 724 can include vapor residue, particles,ions, or clusters of metal, such as tin, lithium, or any other substancethat, when converted to a plasma, emits EUV light. As shown in FIG. 7C,the radicals 705 recombine with the debris 724 to remove the debris 724from the surface 722. The recombination creates a cleaned region 728that is free of the debris 724. The cleaned region 728 can be a circulararea having a diameter of, for example, 6 inches (15.24 cm) or greater.

The conduit 750 is made of a material that does not react or combinewith the free radicals or one that has a low recombination coefficient(for example, a recombination coefficient of about 5×10−3 or less). Therecombination coefficient is a measure of the probability that a radicalwill recombine with another radical that resides on surface after asingle collision with that surface. In the context of the free radicals705 that travel through the conduit 750, the recombination coefficientof the material on an inner wall and the openings 752 and 754determines, in part, the portion of radicals generated by the source 710that reach the element 720. Materials that have a lower recombinationcoefficient allow a larger portion of the generated radicals 705 toreach the element 720 because relatively few of the free radicals 705generated by the source 810 are lost by recombination through collisionswith inner walls of the conduit 750.

The conduit 750 can be made of Teflon, quartz, or glass such asborosilicate glass (for example, Pyrex). In some implementations, theconduit 750 can be made of a metal that is coated, with a material thathas a low recombination coefficient, at the portions that could comeinto contact with a free radical 705. For example, the conduit 750 canbe an aluminum conduit having an interior surface and ends that arecoated with glass, such as, for example, Pyrex. In another example, theconduit 750 can be made of an oxidized metal, such as silicon dioxide(SiO2), titanium oxide (TiO2), or aluminum oxide (AlO2). As yet anotherexample, the conduit can be made of an anodized metal, such as anodizedaluminum. Although a conduit made from a metal oxide can have a higherrecombination coefficient than one made from a non-metallic material, ametal conduit can be relatively easier to machine and can be morerugged.

The material of the conduit 750 and the mass flow rate of the radicals705 through the conduit allow the conduit 750 to be long enough todeliver the radicals 705 to the element 720 in-situ, that is, while theelement is inside of the vessel 730. For example, the conduit 750 canhave a longitudinal extent along the direction 707 of 0.8-2 meters.

Referring to FIGS. 8A-8C, a block diagram of another example of a freeradical transport system 800 is shown. FIG. 8A shows a side view of thetransport system 800, FIG. 8B shows a view of the transport system 800taken along line 8B-8B of FIG. 8A, and FIG. 8C shows a view of thetransport system 800 taken along line 8C-8C of FIG. 8A. The free radicaltransport system 800 can be used in the system 200 (FIG. 2) as the fluidtransport system 202.

The free radical transport system 800 includes a conduit 850 thatdefines openings 854 a-854 l through which free radicals 805 exit theconduit 850 and are directed toward an element 820. The variable sizeand particular placement of the openings 854 a-854 l allows the system800 to provide radicals to the element 820 at a uniform rate, therebycleaning the element 820 at a uniform rate. The sizes of the openings854 a-854 l increase in the direction 807, with the smallest opening(opening 854 a) being closest to the source 810 and the largest opening(opening 854 l) being farthest from the source 810.

Referring to FIG. 8A, the free radical transport system 800 includes asource 810 that produces the free radicals 805. FIG. 8A shows theconduit 850 from the side, with the openings 854 a-854 l orientedperpendicular to the direction 807, so that the free radicals 805 flowout of the openings 854 a-854 l in a direction 808 toward the element820. The conduit 850 defines an opening 852 that couples to the source810 and receives the generated free radicals 805. The conduit 850 passesthrough a sidewall 836 and a sealed port 834 of a vessel 830 (forexample, a vacuum chamber) and into an interior 832 of the vessel 830.The free radicals 805 travel in a direction 807 in the conduit 850 andexit through the openings 854 a-854 l toward the element 820. Theconduit 850 is located a distance 826 from the element 820. For anelement, such as the element 820 that has a curved surface, that facesthe conduit, the distance 826 is the largest distance from the conduitto the element. The distance 826 can be, for example, 15-30 cm.

The element 820 is in the interior 832 a vessel 830. The vessel 830 ispart of an LLP EUV light source (such as the light source 600 of FIGS.6A and 6B). The element 820 defines a surface 822 that is in the path ofplasma that is generated in the vessel 830, and the plasma generationcan cause debris 824 to form on the surface 822. The debris can include,for example, vapor residue, particles, and ions formed from tin dropletsthat are part of the target mixture used to generate the plasma.

When the radicals 805 reach the debris 824, the radicals 805 combinewith the debris 824, thereby removing the debris 824 from the surface822. Because the conduit 850 delivers the free radicals 805 to theelement 820, there is no need to remove the element 820 from the vessel830 for cleaning. Instead, the element 820 is cleaned while residinginside of the vessel 830. Cleaning the element 820 without removal fromthe vessel 830 reduces system downtime because, for example, the element820 is not disturbed and does not have to be realigned after cleaning.

Referring also to FIGS. 8B and 8C, the conduit 850 includes openings 854a-854 l, each of which forms a passage through a wall 856 of the conduit850. FIG. 8B shows a view of the conduit 850 looking upward at theconduit 850 from the element 820, in a direction that is opposite to thedirection 808. FIG. 8C shows a view looking downward onto the element820, with the direction 808 going into the page. FIG. 8C also showsprojections 829 a-829 l of each of the openings 854 a-854 l,respectively, on the surface 822 of the element 820. Together, theprojections 829 a-829 l define a swath 828, which is the portion of theelement 820 that is exposed to the free radicals 805 emitted from theopenings 854 a-854 l.

The sizes of the openings 854 a-854 l increase in the direction 807,with the smallest opening (opening 854 a) being closest to the source810 and the largest opening (opening 854 l) being farthest from thesource 810. As discussed below, the increasing size of the openings 854a-854 l in the direction 807 causes the free radicals 805 reach theswath 828 at a uniform rate.

The source 810 can be a microwave plasma generator. To produce freeradicals with such a source, a gas from which the free radicals areformed is provided to the source 810. For example, to produce hydrogenradicals, hydrogen gas (H2) is provided. An additional gas can be addedto the gas prior to providing the gas to the source 810. For example, amixture of argon and oxygen (Ar/O2) can be added to the hydrogen gas.Both the hydrogen gas and the additional gas mixture have a mass flowrate and velocity when provided to the source 810. For example, thehydrogen gas can be provided to the source 810 at a mass flow rate of 3standard liters per minute (SLM), and the Ar/O2 mixture can be providedto the source at a mass flow rate of 21 standard cubic centimeters perminute (SCCM).

The hydrogen and Ar/O2 gasses enter the source 810, are used to producehydrogen free radicals and a moving gas that passes into the conduit 850in the direction 807. The moving gas that travels in the conduit givesrise to a back pressure that acts along a direction opposite from thedirection 807. The back pressure is a resistance that is encountered bythe moving gas as it flows through the conduit 850.

When present, back pressure has the effect of reducing the mass flowrate or velocity of the gas that travels in the conduit 850, which, inturn, causes the free radicals carried by the gas to stay in the conduit850 for a longer period of time. The time that the free radicals 805 arein the conduit 850 is the “residence time.” The presence of backpressure can result in a higher residence time for the hydrogenradicals, and the higher residence time leads to more opportunities forthe radicals to recombine with the interior walls of the conduit 850 andfewer hydrogen radicals reaching the element 820.

Increasing the mass flow rate or velocity of the gas flowing in theconduit 850 in the direction 807 can also increase the speed at whichthe free radicals 805 travel in the conduit 850 quickly, therebydelivering the free radicals to the element 820 at a higher rate andcleaning the element 820 more quickly. However, increasing the mass flowrate or velocity of the flowing gas also can increase the back pressure,which can lower the amount of free radicals delivered to the element 820(through increased recombination during the radicals' increasedresidence time in the conduit) and can also lower the speed at which theradicals are delivered to the element 820 (through the reduced velocityof the flowing gas caused by the back pressure). As such, increases inthe mass flow rate or velocity of the gas flowing in the conduit 850 arebalanced against the creation of back pressure.

Additionally, in a conduit that lacks openings in a sidewall, thepressure or resistance encountered by gas flowing in the conduit canincrease in the direction of gas flow. If the openings in the sidewallsare all the same size, more radicals 805 exit from the opening closestto the source than any other opening because the pressure inside theconduit 850 is increased towards the source 810. Further, in thissituation, the radicals 805 decrease in velocity as they move throughthe conduit 850 in the direction 807 because of the reducing ordecreasing mass flow in the direction 807. As a result, if all of theopenings in the conduit sidewall are the same size, radicals 805 reachthe swath 828, but the velocity of the radicals 805 reaching the swath828 may not be constant throughout the various portions of the swath828.

In contrast, the openings 854 a-854 l of the conduit 850 have differentsizes, and the sizes of the openings increase in the direction of thegas flow (the direction 807). This arrangement reduces the effectdiscussed above. Thus, when a conduit that has variable sized openingsin a sidewall, such as the conduit 850, is used to deliver the radicals805 to the surface 822, all of the portions of the swath 828 are cleanedat the same rate.

In some implementations, the backpressure in the conduit is kept below0.9-1.2 torr, and the centerline mass flow rate of the flowing gas inthe direction 807 is between 1-4 SLM. The etch rate or rate of removalof the debris 824 can be, for example, 5-125 nanometers per minute(nm/min). The rate of removal can be greater than 125 nm/min. Thecenterline mass flow rate of the flowing gas in the direction 807 can bedefined by a flow pattern. Thus, by applying different flow patterns tothe transport system 800, the centerline mass flow rate can be varied.

The openings 854 a-854 l can have circular cross sections with diametersof 4.5-6.5 mm and can be spaced equidistant from each other on theconduit 850 in the direction 807. The longitudinal spacing between eachof the openings along the direction 807 can be, for example, 40 mm.Although the example shown in FIGS. 8A-8C includes twelve openings, inother examples, more or fewer openings can be used. For example, theconduit 850 can define ten openings or more than twelve openings.

Referring to FIGS. 8B and 8C, to enlarge the area on the element 820that is cleaned, the conduit 850 can be rotated about a longitudinalaxis 859 that is defined by the conduit 850 and extends along adirection that is parallel to the direction 807. Alternatively oradditionally, the conduit 850 can be translated back and forth along adirection 809.

Furthermore, the conduit 850 can be moved in the direction 808 to bringthe conduit 850 closer to the element 820 or in the direction oppositeto the direction 808 to move the conduit 850 further from the element820. Moving the conduit 850 closer to the element 820 can allow a largerportion of the free radicals 805 to reach a particular region of thesurface 822 of the element 820. For example, the conduit 850 can bemoved toward the surface 822 in the direction 308 to direct freeradicals 850 toward a stagnation region. Moving the conduit 850 furtherfrom the surface 822 of the element 820 can enlarge the area that iscleaned by the free radicals 850. The motion of the conduit 850 can bespecified in a flow pattern that defines how the free radicals 850 aredirected toward the surface 822.

The system 800 includes a positioning mechanism 840 that allows a userof the system 800 to move the conduit 850. The positioning mechanism 840can be configured for manual operation, for example, with a lever,wheel, or other mechanical device that is accessible from outside of thevessel 830 and allows the user to move the conduit 850. The positioningmechanism 840 can be computer controlled. For example, the conduit 850can be coupled to a stepper motor or other device that moves the conduit850 when activated by a user or an automated electronic process. Whenthe transport system 800 is used as the fluid transport system 203 (FIG.2), the positioning mechanism 840 is part of or controlled by theconduit controller 240.

Translation of the conduit 850 back and forth along the direction 809moves the conduit 850 relative to the element 820 while keeping thedistance 826 (FIG. 8A) constant. In other words, the conduit 850 movesrelative to the element 820 in a plane that is parallel to a plane thatincludes a perimeter or edge 827 of the element 820. Rotating theconduit 850 about the axis 859 allows the free radicals 805 to bedelivered to regions of the element 820 that are outside of the swath828, such as the regions 821 a and 821 b. Translating the conduit 850back and forth along the direction 809 also allows free radicals 805 tobe delivered to regions of the element 820 that are outside of the swath828.

Referring to FIG. 9, a flow chart of an example process 900 for cleaningan optical element in a vacuum chamber of an EUV light source withoutremoving the element from the vacuum chamber is shown. The process 900can be performed with any free radical transport system disclosedherein. For example, the process 900 can be performed with the transportsystem 700 or 800. In the discussion of the process 900, the system 800is used as an example.

Referring again to FIGS. 8A-8C, free radicals 805 are received at thefirst opening 852 defined by the conduit 850 (910). The free radicalscombine with debris on an element to be cleaned. The debris can begenerated when a target mixture is converted to plasma that emits EUVlight, and the debris can accumulate on a surface of the element byvirtue of the element being in the path of the plasma. The free radicals805 are generated by the source 810. The source 810 can be, for example,a microwave plasma generator, such as the SMART POWER GENERATORavailable from MKS Instruments, Inc. of Andover, Mass. The source can beoperated at, for example, 8000 Watts.

To generate the free radicals, a gas that is capable of beingdissociated into free radicals is provided to the source 810. The gascan be or include, for example, hydrogen (H2), hydrogen iodide (HI),bromine (Br2), chlorine (Cl2), iodine, (I2), methane, or water. Anadditional gas (such as a mixture of argon and oxygen) can be addedbefore providing the mixture to the source 810. The gas is provided tothe source 810 at a mass flow rate or velocity, and the free radicalsgenerated at the source 810 flow into the conduit 850 with a gas flowfrom the source 810. The mass flow rate or velocity can be defined by aflow pattern stored in the flow pattern store 246. Thus, the mass flowrate or velocity can be varied through selection of a particular flowpattern.

The opening 852 and the portions of the conduit 850 that transport orare otherwise in the path of the free radicals 805 are made from amaterial that has a low recombination coefficient. The opening 852 iscoupled to an applicator of the microwave plasma generator so that theopening 852 receives the free radicals 805. The applicator of themicrowave plasma generator can be made of sapphire, and the conduit 850and the opening 852 can couple to the sapphire applicator so that thefree radicals do not encounter any metal surfaces while flowing from thesource 810 into the conduit 850. Such an arrangement can help reduceradical loss at the coupling.

The free radicals 805 in the conduit 850 are directed toward theopenings 854 a-854 l (920). The free radicals 805 can be directed towardthe openings 854 a-854 l by the gas that flows from the source 810 inthe conduit 850. Additionally or alternatively, the pressure in theinterior 832 of the vessel 830, which is where the openings 854 a-854 lprovide a passage to, is lower than the pressure at the source 810 andin the conduit 850. For example, the pressure in the inside of thevessel 830 can be 300 mtorr (40 pascals). As a result, the free radicals805 are drawn from the conduit 850, through the openings 854 a-854 l,and into the interior 832.

As discussed above, to help promote transport of the radicals, theconduit 850 is made from a material that has a low recombination ratewith the free radicals that flow into the conduit 850. Additionally, themass flow rate or velocity of the gas that carries the radicals in theconduit is increased as much as possible while minimizing the effects ofback pressure. Increasing the velocity at which the free radicals 805move in the conduit 850 also reduces the amount of time the freeradicals 805 are in the conduit 850, lessening the amount of radicalloss that is attributable to collisions with the interior walls of theconduit 850. Increasing the velocity of the free radicals 805 alsoincreases the rate of cleaning of the element. In some implementations,the free radicals 805 travel in the conduit 850 at a constant mass flowrate of 1-4 SLM over the longitudinal extent of the conduit 850. Thelongitudinal extent of the conduit 850 can be, for example, 0.8-2meters.

The free radicals 805 are passed through at least one of the openings854 a-854 l and toward the surface 822 of the element 820 (930). Asdiscussed above a pressure differential between the interior 832 of thevessel 830, the source 810, and the interior of the conduit 850, withthe pressure being lowest in the vessel 830, can cause the free radicals805 to pass through the openings 854 a-8541. The openings 854 a-541 areoriented towards the swath 828 and direct the free radicals 805 to theswath 828. The radicals 805 combine with debris 824 on the swath 828 andremove the debris 824. The radical can combine with the debris by, forexample, etching, combusting, or reacting with the debris 824. Theradicals 805 can remove the debris at a rate of 5-125 nm/min.

In some implementations, such as shown in FIGS. 8B and 8C, the openings854 a-354 l can be oriented toward the element 820 by rotating and/ortranslating the conduit 850 so that the openings 854 a-354 l are pointedtoward a particular part of the element 820.

FIGS. 10-13 show other example conduits 1050-1350, respectively. Any ofthe conduits shown in FIGS. 10-13 can be used in the transport system700 or 800 or as the conduit 250 in the fluid transport system 202.

Referring to FIG. 10, an example conduit 1050 is shown. The conduit 1050defines a longitudinal axis 1059 and two ends, a source end 1060 and avessel end 1061. The conduit 1050 has a longitudinal extent 1063 that isthe distance between the source end 1060 and the vessel end 1061 in adirection that is parallel to the longitudinal axis 1059. The extent1063 can be 0.8-2 m. For example, the extent 1063 can be 0.8 m, 0.9 m,0.95 m, 0.975 m, or 1 m. The conduit 1050 has a wall 1067 that definesan outer surface 1056, an inner surface 1065, and a rim 1066. The rim1066 of the source end defines an opening 1052 that has a diameter of1064. The diameter 1064 can be, for example 2.5 cm.

The wall 1067 defines openings 1054 a-1054 p, each of which pass throughthe wall 1067 to form a passage that allows fluid and free radicals topass from an interior of the conduit 1050 to an exterior of the conduit1050. The size of the openings 1054 a-1054 p can vary, with the sizeincreasing along the direction 1007. That is, the opening 1054 a is thesmallest opening, and the opening 1054 p is the largest opening. Theopenings 1054 a-1054 p can be circular in cross section, and can havediameters ranging between 4.5-6.5 mm. The openings can be spaced fromeach other 20-40 mm in the direction 1007. Further, the conduit 1050 canhave more or fewer openings than the example shown in FIG. 10.

The rim 1066 and the inner surface 1065 are made from and/or coated witha material that has a low recombination coefficient. The rim 1066 andthe inner surface 1065 can be or be coated with, for example, Pyrex,quartz, glass, a native oxide (such as silicon dioxide or titaniumdioxide), or an anodized metal, such as anodized aluminum. The rim 1066and the inner surface 1065 can be any material that has a recombinationcoefficient of about 5×10−3 or less. In this manner, the rim 1066 andthe inner surface 1065 recombine with relatively few free radicals,instead transporting the free radicals through the conduit anddelivering the free radicals to an element to be cleaned. In othercases, material with a recombination coefficient of 1×10−2 or higher canalso be used with a corresponding decrease in the cleaning rate.

In use, the source end 1060 is coupled to a source of free radicals andreceives free radicals at the opening 1052. For example, the source end1060 can be coupled to an applicator of a microwave plasma generator.The applicator of a microwave plasma generator is an element thatconverts microwave energy to plasma. The applicator of a microwaveplasma generator can be a tube that is made of, for example, sapphire.Coupling the sapphire applicator tube to the opening 1052 allows thefree radicals generated by the source to flow into the conduit 1050without encountering metal or other elements that recombine with thefree radicals. As a result, the coupling of the conduit 1050 to thesource results in the loss of few, if any, free radicals. The freeradicals travel into the conduit 1050 from the source and exit theconduit through the holes 1054 a-1054 p.

FIGS. 11 and 12 show other example conduits 1150 and 1250, respectively.The conduits 1150 and 1250 can be used in any of the transport systemsdisclosed herein, for example the system 700 or 800 or the system 200discussed above.

The conduits 1150 and 1250 are similar to the conduit 1050, except theconduits 1150 and 1250 have openings that are offset at different anglesby being positioned to pass through different points on a sidewall 1167,1267 of the conduit 1150, 1250. The positioning causes the radicals thatare emitted from the conduits 1150 and 1250 to be emitted over a largerarea of the element to be cleaned. In other words, when projected on tothe element to be cleaned, the openings of the conduits 1150 and 1250cover a larger area than the openings of a conduit that has openingsthat are all oriented at the same angle relative to the element to becleaned. An example of such a conduit is one in which all of theopenings are aligned along a line that is parallel to a longitudinalaxis of the conduit (such as the conduit 1050).

Referring to FIG. 11, the conduit 1150 has a source end 1160 and avessel end 1161. The conduit 1150 has a wall 1167 that defines alongitudinal axis 1159, an outer surface 1156, an inner surface 1165,and a rim 1166. The rim 1166 of the source end 1160 defines an opening1152 that has a diameter of 1164. The diameter 1164 can be, for example2.5 cm. The wall also defines openings 1154 a-1154 s. The openings 1154a-1154 s are arranged in a spiral arrangement on the wall 1157. Theopenings 1154 a-1154 s can be arranged on only one half of the conduit,as shown in FIG. 11. In other implementations, the openings 1154 a-1154s can be arranged on the surface of the entire conduit 1150, so thatradicals are emitted from the conduit 1150 in all directions.

The conduit 1150 has an extent 1163 in a direction that is parallel tothe longitudinal axis 1159. The extent 1163 can be 0.8-2 m. For example,the extent 1163 can be 0.8 m, 0.9 m, 0.95 m, 0.975 m, or 1 m. Similar tothe conduit 1050, the inner surface 1165 and the rim 1166 of the conduit1150 are a material that has a low recombination coefficient.

Referring to FIG. 12, the conduit 1250 has a source end 1260 and avessel end 1261. The conduit 1250 has a wall 1267 that defines alongitudinal axis 1259, an outer surface 1256, an inner surface 1265,and a rim 1266. The rim 1266 of the source end 1260 defines an opening1252 that has a diameter of 1264. The diameter 1264 can be, for example2.5 cm. The wall also defines a plurality of openings 1254, each ofwhich provides a passage between the interior and exterior of theconduit 1150 for radicals and gas. The openings 1254 are arranged incolumns that extend parallel to the longitudinal axis 1259. The openings1254 have different sizes, with the sizes increasing in the direction1207. The example conduit 1250 shown in FIG. 12 has three columns ofopenings. However, more or fewer columns of openings can be used.

The conduit 1250 has an extent 1263 in a direction that is parallel tothe longitudinal axis 1259. The extent 1263 can be 0.8-2 m. For example,the extent 1263 can be 0.8 m, 0.9 m, 0.95 m, 0.975 m, or 1 m. Similar tothe conduit 1050, the inner surface 1265 and the rim 1266 of the conduit1250 are a material that has a low recombination coefficient.

Referring to FIG. 13A, another example conduit 1350 is shown. Theconduit 1350 can be used as a conduit in any of the transport systemsdisclosed herein. For example, and referring also to FIG. 13B, theconduit 1350 can be used in place of the conduit 850 in the transportsystem 800. The conduit 1350 is similar to the conduit 1050, except thatthe conduit 1350 has a radius of curvature 1370. Because of the radiusof curvature, the conduit 1350 has a linear portion 1371 that defines alongitudinal axis 1359, and a curved portion 1372 that bends away fromthe linear portion 1371 and extends along an axis 1373. The radius ofcurvature 1370 can be any curvature such that the angle “A” between thelongitudinal axis 1359 and the axis 1373 is greater than 0 degrees (°)and no more than 90°.

The conduit 1350 includes a source end 1360 and a vessel end 1361. Theconduit has an extent 1374 along a direction 1307 that is parallel tothe longitudinal axis 1359. The extent 1374 can be 0.8 m, 0.9 m, 0.95 m,0.975 m, or 1 m.

The conduit 1350 has a wall 1367 that defines a longitudinal axis 1359,an outer surface 1356, an inner surface 1365, and a rim 1366. The rim1366 of the source end 1360 defines an opening 1352 that has a diameterof 1364. The diameter 1364 can be, for example 2.5 cm. The wall alsodefines a plurality of openings 1354, each of which provides a passagebetween the interior and exterior of the conduit 1350 for radicals andgas. The openings 1354 have different sizes, with the sizes increasingin the direction 1307.

Similar to the conduit 1050, the inner surface 1365 and the rim 1366 ofthe conduit 1350 are a material that has a low recombinationcoefficient.

Referring to FIGS. 14A and 14B, a block diagram of another example freeradical transport system 1400 is shown. The free radical transportsystem 1400 can be used as the fluid transport system 202 of FIG. 2.FIG. 14A shows a side view of the transport system 1400. FIG. 14B showsa cross-sectional view of the transport system 1400 taken along the line14B-14B of FIG. 14A.

The transport system 1400 includes a manifold 1450 (FIG. 14B) thatincludes a plurality of conduits 1450 a-1450 g. The plurality ofconduits 1450 a-1450 g is connected to a source of free radicals 1405.In some implementations, each conduit 1450 a-1450 g is connected to aseparate source of free radicals. The source 1410 produces free radicals1405 that enter the conduits 1450 a-1450 g at openings 1452 a-1452 g,respectively, and flow in the conduits 1450 a-1450 g in a direction1407.

Each of the conduits 1450 a-1450 g has openings 1454 a-1454 l, all ofwhich form a passage through a wall of the conduit and release freeradicals 1405 toward the element 1420. Thus, as compared to a transportsystem that includes a single conduit, the transport system 1400 canclean a larger area of an element 1420 without having to rotate ortranslate the manifold 1450. However, in some implementations, themanifold 1450 can be rotated or translated to further increase the sizeof the region that is cleaned by the free radicals 1405.

Further, in the conduits 1450 a-1450 g, the openings 1454 a-1454 l areeach of a different size, with the sizes increasing in the direction1407. This is similar to the openings 854 a-854 l of conduit 850 thatare discussed above a with respect to FIGS. 8A-8C. As such, the conduits1450 a-1450 g deliver the free radicals 1405 to the element 1420 at auniform rate, which results in debris being cleaned from the element1420 at a uniform rate. In other implementations, the openings 1454a-1454 l can all be the same size. Although the manifold 1450 includesseven (7) conduits, more or fewer conduits can be used. Additionally,the conduits of the manifold can include more or fewer openings thanshown, and the conduits can include different numbers of openings.

Referring to FIG. 15A, a block diagram of another example free radicaltransport system 1500 is shown. The view of the transport system 1500 isdownward toward an element 1520 to be cleaned. The transport system 1500can be part of an LPP EUV light source, such as the source 600 of FIG.6A, and, in such an implementation, the element 1520 is in the interior1532 of a vessel 1530. Additionally or alternatively, the free radicaltransport system 1500 can be used as the fluid transport system 202 ofFIG. 2.

The transport system 1500 includes conduits 1550 a, 1550 b, each ofwhich connects to a source of free radicals 1510 a, 1510 b,respectively. Free radicals from the sources 1510 a, 1510 b flow intothe conduits 1550 a, 1550 b. Similar to the conduit 1050 discussedabove, the conduits 1550 a, 1550 b are made from a material that has alow recombination rate.

The element 1520 defines a surface 1522 that accumulates debris 1524 bybeing in the path of plasma that is generated in the vessel 1530. Theplasma can be generated by passing an amplified light beam through anaperture 1519 in the element 1520 to irradiate a target mixture (notshown) and convert the target mixture to plasma. The element 1520 canbe, for example, a collector mirror that receives EUV light emitted bythe plasma and focuses the light to a location that is out of the pageof FIG. 15A. Thus, in FIG. 15A, the optical path of the amplified lightbeam and the focused EUV light is in a direction that is out of the page(opposite to the direction 1508, which is into the page). The opticalpath of the amplified light beam is out of the page, but also passesthrough the aperture 1519.

As compared to the transport systems shown in FIGS. 8A-8C, 13B, 14A, and14B, the conduits 1550 a and 1550 b of the transport system 1500 areoutside of the optical path. Like the transport system 800, thetransport system 1500 can be used to clean the element 1520 “in-situ,”or while the element 1520 is inside of the vessel 1530. Additionally,because the transport system 1500 is out of the optical path, thetransport system 1500 can be used while the light source is inoperation.

The conduits 1550 a, 1550 b have linear portions 1580 a, 1580 b andcurved portions 1581 a, 1581 b. The curved portions 1581 a, 1581 bfollow an edge 1527 of the element 1520, avoiding the optical path. Theconduits 1550 a, 1550 b are made of a material that has a lowrecombination coefficient and thus delivers free radicals from thesources 1510 a, 1510 b to the interior of the vessel 1530.

Referring also to FIG. 15B, which shows the curved portion 1581 a inmore detail, the curved portion 1581 a defines openings 1554 a-1554 kthat pass free radicals and gas from an interior of the conduit 1550toward the element 1520. The openings 1554 a-1054 k have differentsizes, and the sizes increase with increasing distance from the source1510 a.

More or fewer openings can be formed in the curved portion 1581 a thanare shown in the example of FIG. 15B. The openings can be arrangedsimilarly to one or more of the arrangements of openings shown anddescribed with respect to FIGS. 8A-8B, 10, 11, and 12. For example, aplurality of rows of openings similar to the collection of openings 1554a-1554 k can be formed in the portion 1581 a. The conduit 1550 b hassimilar openings in the portion 1581 b, oriented to direct free radicalstoward the element 1520.

Referring to FIG. 16, another example free radical transport system 1600is shown. The free radical transport system 1600 includes a conduit 1650that follows an edge 1627 of an element 1620 to be cleaned. Like thetransport system 1500 of FIG. 15A, the transport system 1600 includes aconduit that is out of the optical path of the amplified light beam thatis used to generate plasma and/or EUV light that is focused by theelement 1620.

The transport system 1600 includes a conduit 1650 that is coupled to asource of free radicals 1610. Like the conduits 1550 a, 1550 b, theconduit 1650 is made of a material that has a low recombinationcoefficient and, thus, transports the free radicals generated by thesource 1610 to the interior 1632 of a vessel 1630 that contains theelement 1620. The conduit 1650 defines openings that pass free radicalstoward the element 1620.

Referring to FIG. 17, a block diagram of an example EUV light source1700 is shown. The light source 1700 includes an element cleaning system1701. Like the transport systems 800, 1500, and 1600, and the cleaningsystem 201, the element cleaning system 1701 can be used to clean anoptical element. In the example of FIG. 17, the optical element is anelement 1720, which is in an interior 1732 of a vacuum vessel 1730. Thecleaning system 201 cleans the element 170 with free radicals 1705“in-situ” (while the element 1720 is in the interior 1732 of the vessel1730 and without having to move the element 1720). As discussed above, afree radical is an atom, molecule, or ion that has an unpaired valenceelectron or an open electron shell, and, therefore, may be seen ashaving a dangling covalent bond. The dangling bonds can make freeradicals highly chemically reactive, that is, a free radical can reactreadily with other substances. Because of their reactive nature, freeradicals can be used to remove a substance (such as debris) from anobject. The free radicals can remove the debris by, for example,etching, reacting with, and/or combusting the debris.

In addition to the free radicals 1705, the element cleaning system 1701also uses gasses 1776 and 1777, which are gasses that do not includefree radicals, to clean the element 1720. Using the gasses 1776 and 1777with the free radicals 1705 can achieve quicker cleaning of the element1720. In FIG. 17, the free radicals 1705 are shown with solid linearrows and the gasses 1776 and 1777 are shown with dashed line arrows.The manner in which the free radicals 1705, the gas 1776, and the gas1777 are directed toward the element 1720 are defined by one or moreflow patterns. The flow patterns allow the characteristics of the flowof the free radicals 1705, the gas 1776, and the gas 1777 to be variedsuch that the element 1720 can be cleaned more quickly.

The element cleaning system 1701 includes a free radical transportsystem 1702, which directs free radicals 1705 through a conduit 1750 toa surface 1722 of the element 1720. The conduit 1750 is out of theoptical path of the light source 1700, thus, the element cleaning system1701 can be operated to clean the surface 1722 of the element 1720 whilethe light source 1700 is in operation (for example, while the lightsource 1700 is producing the EUV light 1716).

The conduit 1750 can be, for example, either of the conduits 1550 (FIGS.15A and 15B), or 1650 (FIG. 16), or the conduit 1750 may have anotherdesign. FIGS. 18A and 18B, discussed below, show an example of a conduitthat can be used in the light source 1700. The conduit 450 shown inFIGS. 4A-4C also can be used in the light source 1700.

The element cleaning system 1701 also includes a control system 1755 anda gas source 1712, which provides a gas 1713 (such as diatomic molecularhydrogen (H2) gas, helium gas (He), and/or argon gas (Ar)) to theinterior 1732 of the vacuum vessel 1730. The control system 1755controls the flow of the gas 1713 and the flow of the free radicals1705. The gas 1713 that enters the interior 1732 flows into a space 1703around the element 1720. The gas flows in the space 1703 as the gas 1776and the gas 1777.

The flow of the gas 1713 and the free radicals 1705 is specified by oneor more flow patterns that are stored in a flow pattern store 246. Theflow pattern or patterns include information that specifiescharacteristics such as the flow rate of the gas 1713 and the freeradicals 1705. The flow patterns allow the control system 1755 or anoperator of the light source 1700 to manage and change thecharacteristics of the gas 1713 and the free radicals 1705 to optimizethe cleaning of the element 1720.

The light source 1700 includes a target material supply apparatus 1727,which delivers target material 1778 to a target location 1704 in theinterior 1732 of the vacuum vessel 1730. A sidewall 1736 defines theinterior 1732. The amplified light beam 610, which is emitted from theoptical source 615, passes through a sidewall 1736 and an aperture 1717of the element 1720 in the z direction to the target location 1704. Theamplified light beam 610 interacts with the target material 1778 to forma plasma 1717 that emits EUV light 1716 and debris 1724. Some of the EUVlight 1716 is reflected by the surface 1722 of the element 1720 asreflected EUV light 1715. The reflected EUV light 1715 exits the vacuumvessel 1730 toward a lithography system 1799.

The free radical transport system 1702 includes a source 1710 and theconduit 1750, which transports free radicals from the source 1710. Theconduit 1750 is coupled to the source 1710 at an opening 1752 that isformed in an end of the conduit 1750. Free radicals generated at thesource 1710 flow into the conduit 1750 through the opening 1752 and exitthe conduit 1750 through an opening 1754 that passes through a sidewall1756 of the conduit 1750. The free radicals 1705 can flow from thesource 1710 into the interior 1732 of the vacuum vessel 1730 due to apressure differential between the interior 1732 of the vacuum vessel1730 and the source 1710 and/or the interior of the conduit 1750, withthe pressure of the interior 1732 being lower than the pressure at thesource 1710 and the pressure in the interior of the conduit 1750. Thefree radicals 1705 that exit the conduit 1750 through the opening 1754are directed toward the surface 1722 of the element 1720 by positioningthe opening 1754 toward to the surface 1722. The conduit 1750 is made ofa material similar to the material from which the conduit 750 (FIGS. 7Aand 7B) is made from. Like the conduit 750, the conduit 1750 can be madefrom, for example, Teflon, quartz, glass, a metal oxide, and/or a metalthat is coated with a material that has a low recombination rate.

The conduit 1750 is positioned in the interior 1732 relative to theelement 1720 with a gap 1797 between the conduit 1750 and the element1720. The gap 1797 is a physical separation between the element 1720 andthe conduit 1750 through which fluid can pass. In other words, at leasta portion of the conduit 1750 does not make physical contact with theelement 1720. Additionally, the conduit 1750 is positioned so that nopart of the conduit 1750 intersects the amplified light beam 610.Furthermore, the conduit 1750 is positioned to not be between thesurface 1722 of the element 1720 and the EUV light 1716 emitted from theplasma 1717. In this manner, the element cleaning system 1701 can beoperated to clean the element 1720 while the light source 1700 producesEUV light 1716.

In the example shown, the element 1720 is held and supported in a fixedlocation in the interior 1732 of the vacuum vessel 1730 by a mount 1718.The mount 1718 has a sidewall 1719 a, which extends into the interior1732 of the vacuum vessel 1730, and a lip 1719 b, which extends from thesidewall 1719 a into a gap 1797. The lip 1719 b extends from thesidewall 1719 a along a different direction than the direction that thesidewall 1719 a extends. In the example shown in FIG. 17, the sidewall1719 a extends along the z direction, and the lip 1719 b extends awayfrom an end of the sidewall 1719 a along the y direction, which isperpendicular to the z direction.

When the element 1720 is in the mount 1718, the space 1703 is formedbetween the sidewall 1719 a and the element 1720 and/or on a side of theelement 1720, which is on a side of the element 1720 other than a sidethat includes the surface 1722. Some of the gas 1713 from the gas source1712 flows through the portion of the space 1703 that is between thesidewall 1719 a and the element 1720. The gas that flows through thisportion of the space 1703 is shown as the gas 1776 in FIG. 17.

The relative orientation of the lip 1719 b and the sidewall 1719 acauses the gas 1776 to be directed toward the surface 1722 of theelement 1720. The lip 1719 b extends away from the sidewall 1719 a froma location that is just outside of a perimeter 1727 of the element 1720toward the center of the element 1720. Thus, the gas 1776 flows from theperimeter 1727 of the element 1720 toward the center of the element1720. The gas 1776 can be referred to as “perimeter gas flow.”

The gas 1713 from the gas source 1712 also flows through the aperture1717 of the element 1720. The gas that flows through the aperture 1717is labeled as gas 1777 in the example of FIG. 17 and can be referred toas “aperture gas flow.” The gas source 1712 is a source of gas that doesnot include free radicals. Although one gas source 1712 is shown in FIG.17, the light source 1700 can include a plurality of gas sources thatdeliver gas to the interior 1732 of the vacuum vessel 1730. For example,one gas source can deliver the gas 1775 and another gas source candeliver the gas 1776.

The element cleaning system 1701 also includes the control system 1755.The control system 1755 controls the position of the conduit 1750relative to the element 1720, and also controls the flow of the gas1776, the gas 1777, and/or the free radicals 1705. For example, thecontrol system 1755 can adjust the flow rates of the gas 1776, the gas1777, and/or the free radicals 1705 relative to each other to decreasethe amount of time required to remove the debris 1724 from the surface1722. In some implementations, the control system 1755 can start or stopthe flow of one of more of the gas 1776, the gas 1777, and/or the freeradicals 1705 to initiate or end a cleaning cycle. The control system1755 can control the flow of the gas 1776, the gas 1777, and/or the freeradicals 1705 using information included in the flow patterns of theflow pattern store 1746.

The example control system 1755 shown in FIG. 17 includes a conduitcontroller 1740, a flow controller 1741, an electronic processor 1742,an electronic storage 1743, and an input/output (I/O) interface 1744.The conduit controller 1740 controls the position of the conduit 1750relative to the element 1720. The conduit controller 1740 can controlthe position of the conduit 1750 (and the opening 1754 through which thefree radicals 1705 exit the conduit 1750) in the x, y, and/or zdirections, and the conduit controller 1740 can rotate the conduit 1750and the opening 1754 relative to the element 1720 and the surface 1722.For example, the conduit controller 1740 can move the opening 1754closer to the element 1720 in the z direction, or the conduit controller1740 can move the conduit 1750 relative to the element 1720 in the x-yplane to direct the free radicals 1750 toward one portion of the surface1722 at a time. The conduit controller 1740 can be used to position theconduit 1750 relative to the element 1720 and the surface 1722 when theconduit 1750 is initially installed in the interior 1732 of the vacuumvessel 1730, after the conduit 1750 has been installed in the interior1732, and/or while the light source 1700 and radical transport system1702 are in use.

The conduit controller 1740 includes components that are electronicallycontrolled. For example, the conduit 1750 can be coupled to a steppermotor or other device that moves the conduit 1750 when activated by auser or an automated electronic process. The conduit controller 1740 caninclude components that are configured for manual operation, forexample, with a lever, wheel, or other mechanical device that isaccessible from outside of the vessel 1730 and allows an operator of thelight source 1700 to move the conduit 1750.

The flow controller 1741 is used to control the flow of the freeradicals 1705 and the gasses 1776 and/or 1777. The flow controller 1741can include a mass flow controller that measures and/or controls theflow of the gas 1776 and/or the gas 1777 and a pressure controller thatcontrols the relative pressure between the source 1710 of free radicalsand the interior 1732 of the vacuum vessel 1730. The mass flowcontroller is connected to the gas source 1712, and the mass flowcontroller includes an input port, an outlet port, a mass flow sensor,and a valve. The mass flow controller adjusts the valve to adjust theflow of gas from the gas source 1712. Alternatively, the mass flow ratesof gas 1776 and gas 1777 can be controlled separately using two separatemass flow controllers and two separate inlet gas systems (not shown).

The pressure controller can control one or more of the pressure of thesource 1710 of free radicals and the pressure in the interior 1732. Thefree radicals from the source 1710 flow into the interior 1732 of thevacuum vessel 1730 when the pressure in the interior 1732 is lower thanthe pressure at the source 1710.

In some implementations, the control system 1755 does not include theconduit controller 1740 and/or the flow controller 1741. For example, insome implementations, the conduit 1750 is permanently positionedrelative to the element 1720 and is not moved after installation orduring use. In these implementations, the control system 1755 can lackthe conduit controller 1740 or the conduit controller 1740 can bedisabled.

The electronic processor 1742 is one or more processors suitable for theexecution of a computer program such as a general or special purposemicroprocessor, and any one or more processors of any kind of digitalcomputer. Generally, a processor receives instructions and data from aread-only memory or a random access memory or both. The electronicprocessor 1742 can be any type of electronic processor.

The electronic storage 1743 can be volatile memory, such as RAM, ornon-volatile memory. In some implementations, the electronic storage1743 can include both non-volatile and volatile portions or components.The electronic storage 1743 stores instructions, perhaps as a computerprogram, that, when executed, cause the processor 1742 to communicatewith other components in the control system 1755. For example, thecontrol system 1755 can receive an input from a human operator or anautomated electronic process at the I/O interface 1744 requesting thatthe flow rate of the gas 1777 be changed, and the processor 1742 cancommunicate a command to the flow controller 1741 to cause the mass flowcontroller to measure or determine the current flow rate of the gas 1777and adjust the flow rate to match the requested flow rate.

The electronic storage 1743 also includes the flow pattern store 1746,which stores the one or more flow patterns. The flow patterns includeinformation that describes or defines the characteristics of the flow ofthe free radicals 1705, the gas 1776, and/or the gas 1777. For example,the flow patterns can include information sufficient to cause the flowcontroller 1741 to adjust the flow rate of any of the free radicals1705, the gas 1776, and the gas 1777. The flow rates of the freeradicals 1705, the gas 1776, and the gas 1777 can be different from eachother. The flow patterns also can include a time duration that specifieshow long to direct the free radicals, the gas 1776, and the gas 1777toward the element 1720 based on a particular flow pattern. In this way,one or more flow patterns can be used to define a cleaning cycle.

The I/O interface 1744 is any kind of electronic interface that allowsthe control system 1755 and/or its components to receive and/or providedata and signals to an operator and/or an automated process running onanother electronic device. For example, the I/O interface 1744 caninclude one or more of a touch screen or a communications interface.

Referring to FIGS. 18A and 18B, top and side block diagrams,respectively, of an example of a fluid transport system 1801 are shown.The fluid transport system 1801 includes conduits 1850 a and 1850 b, andsources of free radicals 1810 a, 1810 b. The fluid transport system 1801also includes a gas source 1812. The conduits 1850 a, 1850 b transportfree radicals 1805 from the respective sources 1810 a, 1810 b toward asurface 1822 of an optical element 1820. The gas source 1812 provides agas that flows through an aperture 1817 of the optical element 1820 as“aperture gas flow” and around a perimeter 1827 of the optical element1820 as “perimeter gas flow.” The gas source 1812 can include aplurality of individual gas supplies. For example, the gas source 1812can include a supply for the gas that flows through the aperture 1817and a separate supply for the gas that flows around the perimeter 1827.

The optical element 1820 can have a mirrored surface that is positionedin the path of EUV light and debris that are generated by a plasma thatis created from an interaction between an amplified light beam (such asthe light beam 610) and a target material (such as the target material1778). In these implementations, the optical element 1820 may bereferred to as a collector mirror. The element cleaning system 1801 isout of the path of the amplified light beam and is not between thesurface 1822 and the EUV light emitted by the plasma. Thus, the elementcleaning system 1801 can be used while an EUV light source that includesthe optical element 1820 is in use and while the light source generatesEUV light.

The fluid transport system 1801 can be used in the EUV light source 1700to clean the collector mirror 1722 of FIG. 17. The element cleaningsystem 1801 includes conduits 1850 a, 1850 b that can be installed in anEUV light source at the time that the EUV light source is manufacturedor as a retrofit of an existing EUV light source.

Alternatively or additionally, the fluid transport system 1801 can beused as the fluid transport system 202 of FIG. 2. In theseimplementations, the conduits 1850 a, 1850 b are used as the conduit250. The sources 1810 a, 1810 b, and the gas source 1812 are used as thefluid supply 210. As discussed with respect to FIG. 2, the opticalsystem 203 can be any type of optical system. Thus, the fluid transportsystem 1801 can be used with an optical system other than an EUV lightsource. Additionally, in implementations in which the optical system isan EUV light source, the element 1820 can be an optical element otherthan a mirror that is in the path of plasma generated by an interactionbetween an amplified light beam and a target material.

The element cleaning system 1801 cleans the element 1820 by deliveringfree radicals 1805, a gas 1876, and a gas 1877 to the surface 1822. Thegasses 1876 and 1877 can be the same type of gas, and the gasses 1876and 1877 can be gasses that do not contain free radicals. The gasses1876 and 1877 are generated by one or more gas sources, such as the gassource 1712 (FIG. 17). In FIGS. 18A and 18B, the free radicals 1805 areshown with solid line arrows and the gasses 1876 and 1877 are shown withdashed line arrows.

The optical element 1820 has a circular cross-section in the x-y plane(FIG. 18A) and the surface 1822 is curved as shown in FIG. 18B. Theoptical element 1820 has an aperture 1817 that passes through theoptical element 1820 in the z direction. The gas 1877 flows through theaperture 1817 in the z direction. The optical element 1820 also has aperimeter 1827. The perimeter 1827 includes the portions of the opticalelement 1820 that are furthest from the center of the aperture 1817,including the circumferential edge of the optical element 1820. In theexample of FIGS. 18A and 18B, the perimeter 1827 has a circular shape inthe x-y plane.

The element 1820 is held by a mount 1818 that includes a sidewall 1819a. A space 1803 is formed between the mount 1818 and portions of theelement 1820. The space 1803 includes a region 1803 a, which is betweenthe sidewall 1819 a and the element 1820, and a region 1803 b, which isat the side 1823 of the element 1820. The space 1803 receives the gasses1876 and 1877 from one or more gas sources (not shown). In someimplementations, a shroud, such as the hollow conical shroud 650 (FIG.6A), that has sides that taper in the z direction to define an openregion can be placed in the space 1803 b in contact with the side 1823and with the open region of the shroud in fluid communication with theaperture 1817. In these implementations, the gas 1877 flows in the zdirection through the open region defined by the shroud before flowingthrough the aperture 1817.

The mount 1818 also includes a lip 1819 b that extends from the sidewall1819 a at an angle. The lip 1819 b extends into a gap 1897, which is anopen region between the perimeter 1827 and a portion of a conduit 1850a, 1850 b that delivers the free radicals 1805 to the surface 1822. Thelip 1819 b surrounds the perimeter 1827 but is displaced, in the zdirection and radially, from the perimeter 1827. In other words, a gasthat flows in the z direction in the space 1803 a interacts with the lip1819 b and flows out of the space 1803 a from the perimeter 1827 towardthe surface 1822.

The element cleaning system 1801 includes two conduits 1850 a and 1850 bthat transport free radicals from respective sources 1810 a, 1810 b offree radicals to the surface 1822 of the element 1820. The conduits 1850a, 1850 b include sidewalls 1856 a, 1856 b, each of which define aninterior of the respective conduit 1850 a, 1850 b. The conduits 1850 a,1850 b are made of a material that does not react or combine with thefree radicals or one that has a low recombination coefficient (forexample, a recombination coefficient of about 5×10−3 or less) so thatfree radicals 1805 can flow in the interiors of the conduits 1850 a,1850 b. For example, the conduits 1850 a, 1850 b can be made of Teflon,quartz, or glass such as borosilicate glass (for example, Pyrex), or ametal that is coated with a material that has a low recombinationcoefficient.

The sidewalls 1856 a, 1856 b of the conduits 1850 a, 1850 b includerespective linear portions 1880 a, 1880 b and curved portions 1881 a,1881 b. In the linear portions 1880 a, 1880 b, the sidewalls 1856 a,1856 b extend along a longitudinal axis that is substantially straight.The linear portion 1880 a is connected to the source 1810 a at anopening 1852 a, which is formed in an end of the linear portion 1880 a,and the linear portion 1880 b is connected to the source 1810 b at anopening 1852 b, which is formed in an end of the linear portion 1880 b.Free radicals generated by the sources 1810 a, 1810 b flow through theopenings 1852 a, 1852 b into the linear portions 1880 a, 1880 b,respectively. The interior of the linear portion 1880 a is in fluidcommunication with the interior of the curved portion 1881 a, and theinterior of the linear portion 1880 b is in fluid communication with theinterior of the curved portion 1881 b. Thus, free radicals that flowinto the linear portions 1880 a, 1880 b can flow into the respectivecurved portions 1881 a, 1881 b.

The sidewalls 1856 a, 1856 b are curved in the curved portions 1881 a,1881 b. The curved portions 1881 a, 1881 b include openings 1854 thatpass through the sidewalls 1856 a, 1856 b to allow free radicals in theinterior of the curved portions 1881 a, 1881 b to exit the conduit 1850a, 1850 b. For simplicity, only one opening 1854 is labeled on each ofthe curved portions 1881 a, 1881 b. However, the curved portions 1881 a,1881 b can include any number of openings 1854 that are spaced from eachother along a curved portion by a center-to-center radial distance “d.”The openings 1854 can have a diameter of, for example, 4.5-6.5millimeter (mm). The openings 1854 can have a circular cross-section.The distance “d” between two of the openings 1754 can be, for example,1-10 centimeters (cm).

Additionally, the openings 1854 can all be the same size, or the size ofthe openings 1854 can vary. For example, the openings can become largeras the distance that a free radical travels from the source 1810 a, 1810b increases. Such an arrangement may allow the free radicals 1805 toexit the conduits 1850 a, 1850 b at the same rate from all of theopenings 1854.

In some implementations, the spacing “d” between openings 1854 can bemade variable along the length of the curved portions 1881 a and 1881 b.In other words, the spacing “d” can be different between any two of theopenings 1854. For example, the spacing “d” between the openings 1854can be made smaller as the free radicals travel distance from thesources 1810 a, 1810 b increases. That is, the openings 1854 can becloser together (the spacing “d” lower) in the part of the curvedportions 1881 a, 1881 b that are away from the linear portions 1880 a,1880 b and farther apart in the parts of the curved portions 1881 a,1881 b that are near the linear portions 1880 a, 1880 b. Thisconfiguration of the openings 1854 can result in the free radicals beingdelivered at the same rate from every unit length of the curved portions1881 a, 1881 b regardless of the distance to the sources 1810 a, 1810 b.In some implementations, a combination of different holes diameters anddifferent spacing “d” between the openings 1854 throughout the length ofthe curved portions 1881 a, 1881 b can also be used to provide radicalsat the same rate from every length of the curved portions 1881 a, 1881 bregardless of the distance to the sources 1810 a, 1810 b.

The curved portions 1881 a, 1881 b are displaced from the element 1820in the z direction, and the gap 1897, which is a space through which gascan flow, is formed between the curved portions 1881 a, 1881 b and theperimeter 1827. In the configuration shown in FIGS. 18A and 18B, thecurved portions 1881 a, 1881 b are in an x-y plane that is parallel toan x-y plane that contains the perimeter 1827, with these two x-y planesbeing separated in the z direction by the gap 1897. However, the curvedportions 1881 a, 1881 b can be positioned in other configurationsrelative to the element 1820. Additionally, either or both of the curvedportions 1881 a, 1881 b can be moved relative to the element 1820 to,for example, optimize cleaning of a particular portion of the surface1822.

In the example of FIG. 18A, each of the curved portions 1881 a, 1881 bforms part of a circle. The radius of the circle formed by both ofcurved portions 1881 a, 1881 b is the same or greater than the radius ofthe element 1820. The radius of curvature of the portions 1881 a, 1881 bis the same or greater than the radius of curvature of the perimeter1827 of the element 1820. Thus, when the curved portions 1881 a, 1881 bare placed above (in the z direction) and radially outside of theperimeter 1827, such as the configuration of FIGS. 18A and 18B, theopenings 1854 can be oriented such that free radicals 1805 flow from aregion near the perimeter 1827 radially inward toward the surface 1822.For example, the curved portions 1881 a, 1881 b, which include theopenings 1854, can be at the perimeter 1827 or at a distance of up to,for example, 10-15 centimeters (cm) from the perimeter 1827.

Additionally, as discussed above, the lip 1819 b of the mount 1818extends into the gap 1897 at an angle from the sidewall 1818 a to directthe gas 1876 radially inward toward the surface 1822. The lip 1819 b isalso above (in the z direction) and radially outside of the perimeter1827. Thus, the gas 1876 flows from the perimeter 1827.

Due to the configuration of the conduits 1850 a, 1850 b, the element1820 can be cleaned by the fluid transport system 1801 while the surface1822 of the optical element 1820 interacts with light.

Referring to FIG. 19, a flow chart of an example process 1900 is shown.The process 1900 is used to remove debris from a surface of an opticalelement. For example, the process 1900 can be used to remove debris froman element that is inside of a vacuum vessel of an EUV light source. Theprocess 1900 removes debris from the element without requiring that theelement be removed from its operating environment and can be used whilethe optical element is in use. For example, in implementations in whichthe optical element is inside of a vacuum vessel of an EUV light source,the process 1900 can be performed while the EUV light source isoperating (for example, while the EUV source is producing EUV light).

The process 1900 can be performed by the electronic processor 242 of thecontrol system 255 (FIG. 2) or the electronic processor of the controlsystem 1755. For example, the process 1900 can be a set of instructions,perhaps a computer program, stored on the electronic storage 242 that,when executed, cause the one or more electronic processors 242 to causevarious components of the control system 255 to interact to clean theelement 220.

The process 1900 is discussed with respect to the fluid transport system1801 (FIGS. 18A and 18B) and the control system 255. However, theprocess 1900 can be performed with any EUV light source that includes anelement cleaning system such as the element cleaning system 1701 (FIG.17).

A first flow pattern is accessed (1910). The first flow pattern can beaccessed from the flow pattern store 246. The first flow patternincludes information sufficient to define the characteristics of a fluidflow. The first flow pattern can be stored in the flow pattern store 246as, for example, an electronic file or as a set of instructions in theform of a computer program. The first flow pattern can be selected froma plurality of flow patterns in the flow pattern store 246. The variousflow patterns in the flow pattern store 246 can each be associated withone or more optical elements, debris types, and/or optical systems, witha particular flow pattern being optimized for certain conditions. Thus,the accessed first flow pattern can be selected based on the shape ofthe optical element 1820, the type of debris expected to be on thesurface 1822, and/or the optical system in which the optical element1820 is used.

One or more of the free radicals 1805, the gas 1877, and the gas 1876are directed toward the surface 1822 based on the accessed first flowpattern (1920).

The first flow pattern indicates whether the free radicals 1805 flow inboth of the conduits 1850 a and 1850 b or in just one of the conduits1850 b. If the first flow pattern indicates that the free radicals 1805flow in just one of the conduits 1850 a, 1850 b, the first flow patternindicates in which conduit the free radicals 1805 flow. Additionally,when the first flow pattern indicates that the free radicals 1805 flowin the conduits 1850 a and 1850 b, the first flow pattern indicates theflow rate of the free radicals 1805 in each conduit. The flow rates ineach conduit 1805 a, 1805 b can be the same as each other or differentfrom each other. In examples in which the first flow pattern indicatesthat the free radicals 1805 flow in both conduits 1850 a, 1850 b at thesame time and at different rates, the first flow pattern can indicatethe relative flow rates in the conduits 1850 a, 1850 b by, for example,a ratio.

The free radicals 1805 are generated by the sources 1810 a, 1810 b,which are coupled to the conduits 1850 a, 1850 b at openings 1852 a,1852 b, respectively. The sources 1810 a, 1810 b can be microwave plasmagenerators. To produce free radicals with such a source, a gas fromwhich the free radicals are formed is provided to the sources 1810 a,1810 b. For example, to produce hydrogen radicals, hydrogen gas (H₂) isprovided. An additional gas can be added to the gas prior to providingthe gas to the sources 1810 a, 1810 b. For example, a mixture of argonand oxygen (Ar/O₂) can be added to the hydrogen gas. Both the hydrogengas and the additional gas mixture have a mass flow rate and velocitywhen provided to the sources 1810 a, 1810 b. For example, the hydrogengas can be provided to the sources 1810 a, 1810 b at a mass flow rate of3 standard liters per minute (SLM), and the Ar/O₂ mixture can beprovided to the source at a mass flow rate of 21 standard cubiccentimeters per minute (SCCM).

The first flow rate can include information sufficient for the sources1810 a, 1810 b to set the mass flow rate between, for example, 3-15 SLM.Furthermore, the sources 1810 a, 1810 b can be separately controlled sothat the flow rates in the conduits 1850 a and 1850 b are different. Forexample, the flow rate of the free radicals 1805 in the conduit 1850 acan be 3 SLM, and the flow rate of the free radicals 1805 in the conduit1050 b can be 15 SLM.

The first flow pattern also can indicate the direction of flow of thefree radicals 1850. The free radicals 1805 are directed to the surface1822 by orienting the openings 1854 relative to the surface 1822. Forexample, the free radicals 1805 can be directed from a perimeter regionof the element 1820 by placing the opening 1854 near the perimeter 1827and orienting the openings 1854 toward a center of the surface 1822. Theconduits 1850 a and/or 1850 b can be placed at the perimeter 1827 of theelement 1820, or the conduits 1850 a and/or 1850 b can be placed abovethe perimeter 1827 in the z direction (FIGS. 18A and 18B). For example,either or both of the conduits 1850 a and 1850 b can be positioned 1-10cm above (in the z direction shown in FIGS. 18A and 18B) an x-y planethat includes the perimeter 1827. In such a configuration, the freeradicals 1805 flow from the outer edge of the element 1820 toward thecenter of the element 1820.

The first flow pattern can indicate the direction of flow of the freeradicals 1850 through information that is sufficient to cause either orboth of the conduits 1850 a, 1850 b to move relative to the surface1822. The conduits 1850 a, 1850 b can be moved by translation, rotation,or in the z direction (FIGS. 18A and 18B). The conduits 1850 a and 1850b can be moved independently of each other. Thus, the first flow patterncan include information that, when applied to the conduit controller 240(FIG. 2), results in the conduit 1850 a and/or the conduit 1850 b movingand the free radicals 1805 flowing from the conduits 1850 a, 1850 b indifferent directions.

In addition to the free radicals 1805, the gas 1877 and the gas 1876also can be directed toward the surface 1822 based on the accessed firstflow pattern. The free radicals 1804, the gas 1877 (which flows throughthe aperture 1817), and the gas 1876 (which flows around the perimeter1827) can be directed at different flow rates due to the first flowpattern. For example, the information included in the first flow patterncan be sufficient for the flow controller 241 to control the gas supply1812 (which can include a plurality of distinct gas supplies) to set oradjust the flow rates of the gas 1877 and the gas 1876. The flow ratesof the gas 1877 and the gas 1876 are individually controllable, and alsocan be controlled separately from the flow rate of the free radicals1805. For example, the flow rate of the free radicals 1805 can be 3-15SLM, the flow rate of the gas 1876 can be, for example, 0-90 SML, andthe flow rate of the gas 1877 can be, for example, 0-90 SML.

Thus, in the example discussed above, the free radicals 1805 flowthrough two different conduits, the conduits 1850 a and 1850 b. Theconduits 1850 a and 1850 b each provide a fluid path to the element1820. The gas 1876 and the gas 1877 also flow through two differentpaths, with the gas 1876 flowing through the aperture 1817 and the gas1876 flowing around the perimeter 1827. The paths through which the gas1876 and the gas 1877 flow are not spatially coincident with the pathsthrough which the free radicals 1805 flow.

Furthermore, the first flow pattern also can indicate the time durationfor which the free radicals 1805, the gas 1876, and/or the gas 1877 aredirected toward the element 1820. For example, the first flow patterncan indicate that the free radicals 1805, the gas 1876, and the gas 1877are to be directed toward the surface 1822 at particular flow rates andin particular directions for 30 minutes to 2 hours.

The free radicals 1805, the gas 1876, and the gas 1877 flow relative tothe surface 1822 and can move debris that is on the surface. Some of thedebris that is not removed by the free radicals 1805, the gas 1876, orthe gas 1877 is transported to other parts of the surface 1822. Astagnation region can form at the surface 1822. The stagnation region isa region at the surface 1822 through which fluid does not flow. Thelocation of the stagnation region is determined by the flowcharacteristics (for example, flow rate, direction, and fluid type) ofthe free radicals 1805, the gas 1876, and/or the gas 1877.

More than one stagnation region can be present at the surface 1822 at aparticular time. A relatively large portion of the transported debrisbecomes trapped in the stagnation region. Moving the stagnation regionto another location on the surface 1822 can result in fluid being ableto flow through the location where the original stagnation region wasformed. In this way, debris that accumulated at the original stagnationregion can be removed from the original stagnation region (and possiblyalso from the surface 1822 altogether). The stagnation region can bemoved by changing the flow pattern.

A second flow pattern is accessed (1930). The second flow pattern isanother flow pattern that is stored in the flow pattern store 246. Thesecond flow pattern includes information similar to the kinds ofinformation included in the first flow pattern, except at least oneaspect of the information is different than the information in the firstflow pattern. For example, the information included in the second flowpattern can be the same as the information included in the first flowpattern, except that the second flow pattern indicates that the flow ofthe gas 1877 is at a different rate than in the first flow pattern.

One or more of the free radicals 1805, the gas 1876, and the gas 1877are directed toward the surface 1822 based on the accessed first flowpattern (1940). When the free radicals 1805, the gas 1876, and/or thegas 1877 are directed toward the element 1820 based on the second flowpattern, a second stagnation region is formed at a different location onthe surface 1822. Because the second stagnation region is in a differentlocation, fluid flows into and through the location at the surface 1822where the first stagnation region was formed. This allows debris to beremoved from the first stagnation region. The debris can be removed fromthe first stagnation region by reacting or combining with the freeradicals 1805, by physical force of the gas 1876 and/or the gas 1877, ora combination of these mechanisms.

Although the example of FIG. 19 discusses accessing two flow patternsfrom the flow pattern store 246, in some implementations, a cleaningcycle includes only one flow pattern that forms a stagnation region ofminimal severity such that the flow pattern can be used throughout acleaning cycle.

Other implementations are within the scope of the following claims.

For example, the radius of curvature of the curved portions 1881 a, 1881b of the conduits 1850 a, 1850 b (FIGS. 18A and 18B) can be differentthan the radius of curvature of the perimeter 1827 of the element 1820.The radius of a circle or partial circle formed by the curved portions1881 a, 1881 b can be smaller than the radius of the element 1820. Inthese implementations, the curved portions 1881 a, 1881 b can bepositioned above (in the z direction) the element 1820 but radiallyinside the perimeter 1827.

The element cleaning system 1801 can include one conduit with a singlecurved portion that is a circle or part of a circle. The elementcleaning system 1801 can include more than two conduits that, whenplaced in proximity to each other, have a shape that is the same as ashape of the perimeter of the element. Each of the conduits of theelement cleaning system 1801 can be connected to a separate free radicalsource or more than one conduit can be connected to a single freeradical source.

The sources of free radicals discussed above, such as the sources 710,810, 1510 a, 1510 b, 1610, 1710, 1810 a, and 1810 b are shown as beingoutside of their respective vacuum vessels. However, in someimplementations, the source of free radicals can be placed inside thevacuum vessel.

In some examples, the sources 1810 a, 1810 b can be operated at ahydrogen pressure of greater than about 1 millibar (mbar), with amicrowave power of 5 kiloWatts (kW) or greater and the mass flow rate ofthe hydrogen gas being greater than 20 SLM. Under these operatingconditions, a relatively large portion (for example, 30% or more) of thehydrogen gas can disassociate into hydrogen radicals that can betransported by the conduits 1850 a, 850 b. Such operating conditions, incombination with the arrangement and geometry of the openings 1854 inthe conduits 1850 a, 1850 b, can provide hydrogen radicals with avelocity of, for example, 1 meter per second (m/s) or greater.

In addition to the conduits 450 a, 450 b, 1850 a, and 1850 b, theconduit 250 (FIG. 2) can be any of the conduits 750, 850, 1050, 1150,1250, 1350, 1450, 1050 a, 1050 b, 1150 a, 1150 b, and 1250.

What is claimed is:
 1. A system comprising: a conduit comprising asidewall, the sidewall comprising at least one opening that passes froman interior of the conduit to an exterior of the conduit, the conduitconfigured to transport free radicals in the interior of the conduit andto pass the free radicals through the at least one opening that passesthrough the sidewall; and a control system comprising an electronicstorage coupled to one or more electronic processors, the electronicstorage comprising instructions that, when executed, cause the one ormore electronic processors to: select a flow pattern from a plurality offlow patterns stored on the electronic storage, the flow patternscomprising free radical flow patterns, each of the free radical flowpatterns comprising information sufficient to describe a flow of thefree radicals through the at least one opening, the informationsufficient to describe the flow of the free radicals through the atleast one opening comprising at least a flow rate of the free radicalsover a time period, and apply the selected flow pattern to a flowcontroller of the system to cause the free radicals to pass through theat least one opening at a flow rate for the time period indicated by theselected flow pattern, wherein the flow patterns further comprise gasflow patterns, the gas flow patterns including information sufficient todescribe a flow of gas, the gas flowing in the system separately fromthe conduit, and the information sufficient to describe the flow of thegas comprising a flow rate of the gas over a gas time period, and theinstructions further comprise instructions that, when executed, causethe one or more processors to apply the selected gas flow pattern to theflow controller of the system to cause the gas to flow in the system ata gas flow rate for a gas time period indicated by the selected gas flowpattern.
 2. The system of claim 1, wherein the conduit is configured forplacement relative to an optical element in a vacuum chamber of anextreme ultraviolet (EUV) light source.
 3. The system of claim 2,wherein the optical element comprises a collector mirror, the collectormirror comprising an aperture, the selected flow pattern comprises afree radical flow pattern and a gas flow pattern, and applying theselected gas flow pattern to the flow controller of the system causesthe gas to flow through the aperture of the collector mirror at the gasflow rate and during the gas time period indicated by the selected gasflow pattern.
 4. The system of claim 3, wherein: the conduit comprisesat least two curved portions, the curved portions having a curvaturethat follows a perimeter of the collector mirror, the selected freeradical flow pattern describes the flow of free radicals through each ofthe at least two curved portions, applying the selected gas flow patterncauses the gas to flow through the aperture of the mirror, and around aperimeter of the mirror, and applying the selected free radical flowpattern causes the free radicals to flow from at least one of the atleast two curved portions.
 5. The system of claim 1, wherein the conduitcomprises a first conduit and a second conduit, each of the firstconduit and the second conduit comprising a sidewall, the first conduitcomprising a main branch, a first branch, and a second branch, the firstbranch and the second branch being in fluid communication with the mainbranch, and the first branch and the second branch being shaped forpositioning along first and second portions of a perimeter of an opticalelement, the first and second portions being different portions of theperimeter of the optical element, and the second conduit comprises acurved sidewall shaped for positioning along a third portion of theperimeter of the optical element, the third portion being different thanthe first and second portions.
 6. The system of claim 1, wherein thecontroller is further configured to move the conduit relative to theoptical element.
 7. A method comprising: accessing a first flow pattern,the first flow pattern comprising information sufficient to describe aflow of free radicals and a gas relative to an optical element;directing the free radicals toward the optical element based on thefirst flow pattern, the free radicals being directed through a pluralityof paths and at different flow rates in at least two of the paths; anddirecting the gas toward the optical element based on the first flowpattern, the gas being directed along a path that is different from anyof the paths along which the free radicals are directed.
 8. The methodof claim 7, further comprising: accessing a second flow pattern, thesecond flow pattern comprising information sufficient to describe a flowof free radicals and a gas relative to an optical element, and thesecond flow pattern having at least one aspect that is different fromthe first flow pattern; directing the free radicals toward the opticalelement based on the second flow pattern; and directing the gas towardthe optical element based on the second flow pattern.
 9. The method ofclaim 8, wherein the at least one aspect comprises a flow rate.
 10. Asystem for an extreme ultraviolet (EUV) light source, the systemcomprising: a fluid transport system comprising: at least a firstconduit and a second conduit, the first conduit being configured toreceive free radicals and gas from a first supply, and the secondconduit being configured to receive free radicals and gas from a secondsupply, the first conduit comprising a sidewall that defines an interiorspace configured to transport free radicals and gas from the firstsupply, and the second conduit comprising a side wall that defines aninterior space configured to transport free radicals and gas from thesecond supply, the sidewalls of the first conduit and the second conduitdefining at least one opening that passes from the interior space of therespective conduit to an exterior of the respective conduit, the atleast one opening being configured to pass free radicals and gas,wherein the first conduit comprises a main branch, a first branch, and asecond branch, the first branch and the second branch being in fluidcommunication with the main branch and the first supply, each of thefirst branch and the second branch being positionable to followdifferent portions of a perimeter of an optical element in a vacuumchamber of the EUV light source, and the second conduit is positionableto follow a different portion of the perimeter of the optical elementthan the first branch and the second branch of the first conduit; and acontrol system configured to control the transport of free radicals andgas in the first conduit and the second conduit.
 11. The system of claim10, further comprising a flow controller, and wherein the control systemis configured to: select a flow pattern from a plurality of flowpatterns, each of the flow patterns comprising information sufficient todescribe a flow of the free radicals through the openings of the firstconduit and the second conduit, the information sufficient to describethe flow of the free radicals through the at least one openingcomprising at least a flow rate of the free radicals over a time period,and apply the selected flow pattern to the flow controller to cause thefree radicals to pass through the at least one opening at a flow ratefor the time period indicated by the selected flow pattern.
 12. Thesystem of claim 11, wherein the flow patterns further comprise gas flowpatterns, the gas flow patterns including information sufficient todescribe a flow of gas, the gas flowing in the system separately fromthe conduit, and the information sufficient to describe the flow of thegas comprising a flow rate of the gas over a gas time period, and thecontrol system is further configured to apply the selected gas flowpattern to the flow controller to cause the gas to flow in the system ata gas flow rate for a gas time period indicated by the selected gas flowpattern.
 13. The system of claim 10, wherein the first conduit and/orthe second conduit are positionable to follow the perimeter of theoptical element in a plane that does not include any part of the opticalelement.
 14. The system of claim 10, further comprising a conduitcontroller configured to move the first conduit and/or the secondconduit relative to the optical element.
 15. The system of claim 12,wherein the flow rate of the free radicals and the gas flow rate in thefirst conduit is separately controllable from the flow rate of the freeradicals and the gas flow rate in the second conduit.
 16. The system ofclaim 11, wherein the control system is further configured to selectmore than one flow pattern from the plurality of flow patterns during acleaning cycle.
 17. The system of claim 16, wherein each of the selectedflow patterns is active during a different portion of the cleaningcycle.
 18. The system of claim 11, wherein the information of at leastone of the plurality of flow patterns comprises a different flow rateand/or a different time period than the information of another of theplurality of flow patterns.
 19. The system of claim 10, wherein thecontrol system being configured to control the transport of freeradicals and gas in the first conduit and the second conduit comprisesthe control system being configured to (i) control a flow rate of freeradicals and/or gas in the first conduit and/or (ii) control a flow rateof free radicals and/or gas in the second conduit.
 20. The system ofclaim 10, wherein the control system being configured to control thetransport of free radicals and gas in the first conduit and the secondconduit comprises the control system being configured to control thetransport of free radicals and gas in the first conduit and the secondconduit independently.